Usually refers to somebody who takes up extra room on public transportation so that other people can't sit next to them.

I have not heard this either. But If i had to guess, it's probably similar to 'Couch Potato'. Seat Pig: a person who has taken on the attributes of a pig because they are always seated.

It's "seat pig", however, what is the meaning of "sit pig"?

Do you mean "seat belt", or "Sit Pig!" ?

I've never heard of it either. Is this something you read or something you heard? Do you have the rest of the sentence?