I'm becoming my body bigger and biggergaining more and more weight. I'm gaining more and more weight.

You can also say "My body is becoming bigger and bigger" but "I'm gaining more and more weight" is more natural.

I'm thinking I want to lose weight untilby the time I starting university, so I recently started walking as a hobby. I want to lose weight by the time I start university, so I recently started walking as a hobby.

You can also say "I'm thinking that I want to lose weight by the time I start university, so I recently started walking as a hobby."

But I eat a lot of foodsHowever, when I walk for a long distance. I end up eating a lot of food. However, when I walk for a long distance I end up eating a lot of food.

But I eat a lot of foods when I walkgo for a long distancewalks. But I eat a lot of food when I go for long walks.

The other suggested correction is fine, too. "However" has a more formal feel that I think doesn't match the rest of your entry.

Also, I like confectionerysweet food very much. Also, I like sweet food very much.

In day to day speech, and writing, not many people use "confectionery".

I don't imagine If I want to get a slim body, so I have to pass over snacks. If I want to get a slim body, I have to pass over snacks.

I don't imagine I get a slim body, so I have to pass overf I am ever going to become slim, I must give up snacks. If I am ever going to become slim, I must give up snacks.

The other correction is fine, too.