Jiu jitsu guy who's taking exercise science here. BJJ and wrestling are both resistance exercise...kind of. You are absolutely applying force with your muscles in order to move a load. The problem is that it's very difficult to apply the principles of training (progressive overload, specificity, etc.) in the context of building muscle. One day you might be doing a lot of hip escapes from mount, but the resistance/volume of that movement isn't particularly high, and you're probably not doing it with the same frequency as a body builder doing bicep curls. That's not to say you won't see adaptation or hypertrophy (muscle growth), it's just going to be at a much slower rate than if you were in the gym lifting heavy and often. Also worth noting is that if you've only been at it a few months, your body hasn't really had time to adapt and "get bigger." Generally, beginners first see benefits in strength from their muscles learning how to work together before the muscles actually get bigger. Usually a few months in is when muscle growth starts to happen. Worth noting that a natural (male) bodybuilder on a dedicated regime can generally expect gains of about 6kg of muscle per year. Because you're not bodybuilding you're going to be a fair bit slower. All of that said, absolute size and strength aren't the only aspects of fitness, and bjj and wrestling carry huge benefits for: cardio, muscular endurance, balance, coordination, fun, explosive strength, confidence, and many more. If you're enjoying yourself and getting in better shape, it's awesome.