Maybe capcom is buying into what western society thinks a hero should be and look like?

As mentioned, he bulked up after his loss to Wesker in CV as he knew he'd eventually have to confront Wesker again and lacking Wesker's Superhuman abilities, the best he could do was increase his own strength.

As far as LiN goes, his outfit is pretty much very similiar to the STARS costume in that it hides his arms and any details on his chest. So while yes, the model is actually the same and has about the same frame and we all know this. You can't physically see his huge chest or his bulging biceps so they can 'semi' get away with it. You can also say he's been training since CV and thus would be bulkier by the time of LiN and after the failure and loss during LiN, he 'redoubled his efforts' as mentioned and worked himself to his RE5 stature.

Again, we all know the models are the same but, going from a story perspective, this seems the most logical. Especially if you as mentioned, let capcom away with the fact you can't see his biceps and chest in the LiN outfit so you can sorta play to the idea that he's not as bulked as RE5 'yet'.

There are four years from CV events (December 1998) to Umbrella's End (February 2003), and Chris is exactly the same from Remake. This makes me think that he doesn't start his fisical training at the time of CV.

In LiN (2006), he looks like if he had the same muscle from RE5, but I too think that this is non-canon. The lazy Capcom just don't changed his model.

So, I think that Chris started his intensive training after Jill's "death", and the result is showed in RE5.

Yup. You nailed it perfectly!

I'm more inclined to say he started the intensive training after the search for Jill was called off.

But wasn't he already that buff when she was alive like you said in the flashback?