Depends. I've (deliberately) put on about 15 pounds. My "skinny" pants that I bought (after having LOST 45-50lbs) still fit fine around the waist, but have gotten tighter around the thigh. Why? Because I've been doing heavy squats. The mirror tells me that the 15 or so lbs I've put on have, in fact, been mostly muscle.

Does your "body weight resistance" include squats?

Lunges are deadly for creating great quads, IME!

Seriously, I couldn't possibly fit into any skinny jeans with my quads and I wear a size 4. Muscle is denser than fat & doesn't "squish" so much, hence your problem. I remember noticing this happening when I first started working out in high school and I no longer fit into my Junior size 3 jeans.

Personally, I'd buy some looser-cut jeans and enjoy my new muscles!

If you are a mesomorph, the only way you are going to ectomorph territory is if you become unhealthy and allow lean tissue loss. Fat is "squishable", so when you build muscle things are going to be tighter.Your body type determines if you can gain muscle without gaining circumferance- for many of us stocky-types, it's either muscles and circumference, or no muscles and squishiness. I've never lost weight exercising- I just build muscle and even if it feels like you are going to look "thicker", believe me you do just end up looking leaner! Even though doing upper body weight makes me build arm muscle like nobodies business, and my arms get thicker (tight sleeves), they look way leaner because they don't squish or change shape when they go by my side. Legs will look like that too. My mom recently gained back 20 lbs of muscle after recovering from a surgery that caused a lot of wasting, and she looks really great and lean even though she's never had so much muscle. I think you should learn to enjoy your muscles, and just observe how your body changes as you exercise!

Consider adding hatha yoga, pilots or bar method to your exercise: all focus on elongating muscles.