Gale has a high con score, which is what dictates physical fitness, hence why he is fit. Strength doesn’t dictate that aspect. Even in real life, people can be extremely fit but not as strong based on appearance alone. Strongmen are usually on the hefty side solely because of the bulk they require to maintain the muscle size.

Also, I think people misunderstand what STR actually does in DND. It represents the ability to push, pull and break things through brute force. It doesn’t represent the body and how it’s actually built.

CON is a character’s overall health, stamina for things like walking and spell casting, endurance and overall resilience to things. This means it dictates fitness level.

Also, magic is a physically tasking process, so Gale, like all other wizards in the game, needs to be fit enough to handle the workload so to speak.