1. Some of those orcs looked kind of small and regardless the clothing looked big on them
2. Sam thought Frodo was dead which was why he took the ring, now that he knows Frodo is alive he can’t take it. Frodo was the ring bearer and he was the one who had to carry it.
1: Correct. In both the book and the film we see various kinds of orcs, including the little bug-eyed ones in Moria. Frodo and Sam took stuff from small orcs, and it was still too big.
2: Sam did take the ring when Frodo was paralyzed and couldn't stop him, but once Frodo was walking and talking again... he'd die before he let Sam take the ring away from him! That's something that was made clear in the book and not so much in the movie, all through Mordor the ring was winning its internal battle with Frodo, Frodo had no will left and as they got closer and closer to Mt. Doom it was increasingly clear that it was all futile, because Frodo would never be able to destroy the thing.
2. If Frodo was incapable of getting his own self up the hill but Sam was capable of getting them both up there, I think Sam could have grabbed the ring and outrun Frodo.
Additionally, if you are right that Sam knew Frodo had basically gone full Gollum at that point, it was his fundamental duty to the entire world to do everything he could to get that ring into the volcano, especially when they were so close. He certainly should not have hung back and given Frodo a chance to turn invisible. He should have stuck right by him with the plan to tackle him and knock them both into the volcano if necessary.
Grabbing the One Ring would have turned the story into Sam vs. Frodo, and after Sam's experience with the ring promising him all kinds of wonders he didn't want to have to fight both Frodo and the ring itself. Plus, Sam loved Frodo, he wanted to help rather than hurt him.
So by carrying Frodo, Sam helped Frodo and avoided conflict with him, he avoided contact with the accursed ring, and he kept their mission going rather than starting a fight that would derail it, etc. Sam not only did the kind thing, he did the *right* thing.
Sam should have been prepared to throw Frodo into the Crack of Doom, and neither the book nor the film said that he did. He was concentrating on getting them both there, it never said that he thought sensibly about what to do once they got there!
If Sam couldn't live with the thought of living on after he killed Frodo, he could have been prepared to tackle the hobbit he loved and and not have to worry about what came next. Because realistically, there was no getting back through Mordor.