I see what you're saying.. They are slim sweatpants and they're supposed to fit a certain way... but at the same time, I doubt most people are wearing them for its intended use. (I know I'm not) I wear them to lounge in, run errands, things like that.. doubt I'll wear them to the gym.
I'll still stick to what I recommend to others from personal experience. Go True to Size for a very FITTED/SLIM silhouette, or you can size up one and be slightly relaxed. Don't get me wrong, sizing up is still pretty slim, its just not tight. I for one workout consistently and have nice legs, so true to size wouldn't of worked under any circumstance.