Im gonna guess your girl is just comfortable in her own skin...and thats the way it'll be. My buddies girl started hitting the gym after a night at a club when a girl walked by and said "wow he is way to hot for her" she started crying and bought a gym pass the following week.
The way I see it is...there are people that never workout, or that workout to show some sort of athleticism(once in a great while), or do it because they feel insecure, or want to be more confident.
My girl is beautiful, shes 5'4 115lbs. 3/4 asian 1/4 black Her base is built well, but her shoulders/arms/back lack really any definition. Being so small, even 5lbs makes a difference in her appearance around the mid section, arms, etc...
She will go once in a while like once or twice in 2 months... Im a everyday gym rat. Ive tried explaining what I perceive as men liking most, or to just bring her body into proportion, her legs are awesome, her butt is athletic but her upper body lacks.
I've read somewhere when couples workout, it even brings out better sexual desire with one another as well. Men/Women need to want the self-improvement...some people have it, some people dont. But also remember when we're in love, or in relationships..we tend to let things go. Ive been through that alley before.
What I don't understand is...lets say You look 10x better than your gf/bf...why doesn't one desire to look as good or even better than you? I noticed when I was ripped, my girl didn't care one bit about her self. However, she did make hints she was slightly more insecure. Either way, I still find her hot as hell.