Not sure if this has been addressed yet but certain armor (such as the BOS combat armor for example) changes shape when worn with different clothing. Is this normal?

I guess so, model adapting to the clothing to avoid clipping issues perhaps?

I was not aware of this. Are you sure you are not conflating different tiers of combat armour, normal, sturdy, heavy?

Different clothing and different mods will both change the look.

For instance, Armored Military Fatigues with all Pocketed Armor will make my character look really, really fat. The Fatigues blow up like a balloon around each limb and the body portion of each expands even more. It looks like I am wearing one of those Inflatable Sumo Suits. Not a good look. So I just use the Minuteman underlayer for the time being.

Yeah thats wierd becouse when I see gunners in military uniforms wearing combat armor (even hvy one) it looks ok on them. But if player character is wearing military fatigues and hvy or strudy combat armor it looks kinda "fat" and bad.

I find outside of wearing the BOS uniforms, or Vault Suits, wearing combat armor on clothing such as military fatigues or other clothing, makes your character look fat and bulky. It svcks. Unfortunately, the combat armor colours don't really matchup well with the vault suit. Best outfit I have found is the BOS Officer Uniform, with combat armor on top. But you have to kill someone to get it, which svcks. Still looking for another combo to use. I'm wearing the BOS normal uniform, with shadowed combat armor on top currently.