This might be a dumb question since I might just be taking the line to literally but Yoda says that "size doesn't matter" but it kinda does? Otherwise wouldn't we see force users lift massive ships with the force? Or how Yoda and other force users are visibly strained while lifting large objects
Yoda was referring to his size, and judging things by their size. That said, even the Force has limits.
I think the point he was making can also be described as, "Big things can step on small things, but small things can fly into the superstructure and destroy the main reactor core."
The issue seems to lie more with the mindset a person might have. "That thing looks heavy, how am I supposed to lift it?" is exactly the mindset that causes you to fail before you even begin. The difference between someone who has complete confidence in his abilities and someone who lacks the confidence and who is already coming up with reasons as for why he couldn't possibly succeed before he even sets out to do it. That doesn't mean that everything is equally hard to lift regardless of size or weight, just that you need to stay away from assuming that something cannot be done just because a regular person might not be able to. And who knows how far you could actually go, just because lifting someething heavy looks taxing, doesn't mean that the force doesn't allow to lift even heavier things. You can be really good at a physically demanding work, look strained while doing it, yet still have plenty of room to do even heavier work. After all, it isn't a black and white thing where you either look strained or you don't. And outside of the universe, you obviously run into the issue that if you make someone too powerful, you generally end up with contrieved ways to stop said person from succeeding. E.g. if a Jedi could crush the Death Star through the force, what kind of threat could you possibly create that would be a match for him? You'd end up with said Jedi either being unwilling to intervene or being missing for some reason, neither is a great solution unless you have a perfect reason for it. That's why Luke personally being on Crait and facing all the First Order walkers would look bad, while him not actually being there and having to deal with the consequences of his action doesn't make him too powerful. If powers get too extreme, you end up in situations where people ask "why did person X not use power A, like person Y did in movie Z?". It's much better to have the possibility of what the force might allow, paired with strong but not way over the top things shown in the movie, than having painted yourself into a corner by showing stuff that gets crazier and crazier.
I think that Yoda was urging Luke to look beyond surface appearances hence why he would use himself as an example, pointing out that Luke doesn't judge him by his size. In a sense, he is encouraging Luke to see beyond the merely physical and glimpse the spiritual and the metaphysical or as he explains, "Luminous beings are we, not this crude matter." Luke is focusing too much on the crude matter, and so Yoda is emphasizing that he should be looking at the galaxy through a more spiritual lens. Yoda is also pointing out that there is an element of mental construct at work in Luke's decision that he can't lift the ship. He thinks that it is too large for him to lift, and therefore, it is too big for him to lift in a sort of self-fulfilling prophecy. We see that when he is able to lift the ship but then the doubts set in, and it crashes into the swamp again, which is why Yoda shakes his head. Luke's limitation then is not a physical barrier--wherein it is impossible for him to use the Force to lift the ship--but one of his mindset, his belief that he cannot use the Force to lift the ship out of the swamp. It is sort of reminiscent of the Biblical quote about faith the size of a mustard seed being able to move mountains. Rationally, we could say quite fairly that it is impossible to move mountains especially merely by faith alone. Yet perhaps the point of the Biblical quote is just to have faith despite the odds, despite the size of the mountain. The size of the mountain or the ship doesn't matter in comparison to faith and trust in a higher power whether that be the Biblical God or the Force is the point of both statements, I believe. On the purely rational level, Yoda's statement doesn't make sense. On a more metaphysical and spiritual level, however, it has resonance. At least in my opinion.
That quote is the main reason why I don't buy all those "Starkiller is OP." arguments. Yoda was definitely referring to the size of the X-Wing in comparison to the small rocks that Luke was lifting. Also remember what Vader said in episode 4 ? "The ability to destroy a planet is insignificant next to the power of the force.”
the line is pretty funny considering it was his size that lost the duel with Sidious. His tiny arm couldn't hold on to the pod to reach up.
On the other hand, Yoda managed to escape via the ventilation ducts, where a human-sized Jedi might easily have been found and killed by Sidious or his stormtroopers.
Yoda said exactly what he needed to say in that instance to get through to Luke. That's all that matters.
I always assumed that the challenge in lifting heavy objects isn't that they are literally heavier in the force, but it is significantly harder to avoid thinking about their weight as you lift them. As long as their mind is on the force, size matters not. But the moment you open your eyes and think "Wow, no way I could lift that", your capacity to focus goes down significantly, and it becomes harder to lift it. The "strain" you mentioned isn't physical exertion at lifting a heavy object, but rather a way of showing focus. They need to clear their minds, think about the force, and ignore the physical weight. Something that is easier said than done.
To a certain limit of course! Lifting an X-Wing is one thing. But do you honestly think even the most powerful Force user could move a star destroyer or Death Star? Highly unlikely.
Give me a lever long enough and a fulcrum on which to place it, and I shall move the world. Archimedes
Yoda said it cause Luke said that the ship was too heavy for him to lift. Luke also didnt think much of Yoda cause of his size(Yoda mind read it) and Luke simply did not truly believe in the power of the Force : Luke: I can't. It's too big. Yoda: Size matters not. Look at me. Judge me by my size, do you? Hmm? Hmm. And well you should not. For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us. Luminous beings are we, not this crude matter. You must feel the Force around you; here, between you, me, the tree, the rock, everywhere, yes. Even between the land and the ship. Luke I don’t believe it( Luke stammers as his fighter now sits on the ground.) Yoda: That is why you fail Also judging by Luke's facial expression as a Force Ghost some 40 or so years later when he did lift the X-Wing in TROS there was a great deal of concentration and weight to it.