He definitely has lost match fitness but I'm not so sure about this 'overweight' business. It seems to me he obviously has wide hips and just carries more skin/baby fat lol, if you know what I mean.
His metabolism is probably not very high/he's not burning enough energy i.e because he's not playing every week. The thing is once he does, you're getting a monster of a physical left back, fast in acceleration, fast in top speed carrying the ball, fast over long distances and a sturdy frame to contend with. That's not to mention, good technique and touch.
The kid is so young, once he finds rhythm to his game attacking wise, you will opposition teams respect that and play off him. That will expand our attacking dimension.
I said it in the Bailly injury thread I think but one 'positive' out of that is Mourinho will be forced to play Shaw more; unless Rojo suddenly jumps in favour (after a good game against City), the automatic first choice back four should be Tony, Smalling, Blind and Shaw.
Those four will have to play their way into form and find the partnerships that help each other play better.
I'm personally excited to see that as I think Shaw, like Herrera, is just perennially on the brink of breaking out. To be fair to Shaw, this would be his 2nd coming after being cruelly stopped by that cnut.