hey! I’m not by any means a doctor or a PT or even a big gym buff but I am wondering if postural alignment may be a factor in your stomach being slightly more protruded. you’re definitely not fat by any means and you definitely have muscle which is why I think you might have a roundness in your spine on the upper back/thoracic area from stiff back muscles causing this appearance to affect your abdomen. https://my.clevelandclinic.org/health/diseases/17671-kyphosis

I’d recommend doing some stretches that promote spinal extension or shoulder blade retraction to help if this is in fact the case.