Hi Mirof,
You raise an interesting point.
Gustav lived from 1594 to 1632. That means he was killed at age 40. Pretty young.
The key portraits seem to suggest an older man, certainly not slim and even rather portly. He appears to have aged considerably with the weight of responsibility and constant warfare.
The casting is of a portly figure. There is a similar portrait of him in a red Polish looking caftan(?). I don't propose to file it down to a slimmer figure. I might shave down the chubby cheeks a little. That's all.
rgds
Victor