There's technically nothing wrong with the sentences you've written. However, in the sense of food on the plate, I think other phrases are more common.
It's a lot for me. -> That's plenty, thank you. (Assuming that the server is still dishing it out and you are saying that the amount of food is enough. You could also say, "It's too much for me," but depending on the situation, that could be considered rude.)
It's not enough for me. -> Can I have some more, please?