I hope it will be explained how Saw Gerrera got that fat in 20 years.
Um, I would assume he ate a lot during said 20 years.
I hardly think that is what requires a backstory here. I’d be more curious as to how his eye colour changed from blue to brown over 20 years. Now that’s something the EU will need to make a contrived explanation for in the future.
I hardly think it would be too much of a stretch to assume that this character was written as an original character for RO then someone at Lucasfilm noticed some similarities to Saw Gerrera and asked them to change the name so that RO could tie in with the EU. I doubt that Gareth Edwards or any of the screenwriters where actively trying to include TCW characters.