My kittens transformed! Is this his winter coat?

This is my first time owning medium-haired cats, so I’m not sure what to expect with their coats. We got our two boys in November when they were 4 months old, and now, at 7 months, one of them looks completely different!

He’s so much fluffier than before, but his brother hasn’t changed nearly as much. Could this just be his winter coat?

First picture is 4 months old, second picture is 7 months right after a brushing, and third is also 7 months after a romp with his brother.