Is Sally was getting Chubby a metaphor?

No.

But this would be:

Sally was a fat pig.

This is a simile:

Sally was fat as a pig

A metaphor states "X is Y" when X is not actually Y but can be compared to it.

A simile states "X is like Y"

"Sally was getting chubby" isn't either.

I feel sorry for poor Sally, though. But at least she didn't become a linguist, and have to deal with these nuisances all day.