Mass has nothing to do with martial arts or athleticism!
Now strength training is a whole different beast all together. Try doing bodyweight exercises. like pull ups, pushups, deep knee bends, handstand pushups, leg raises,wrestler bridges. Those are the 6 groups you need to work on also throw in something like burpees or kettle bell swings. These exercises and its variants hits every part of your body. This is a solid foundation to build from spend 6 months doing that and go from there.
With the bodyweight you get freakishly strong and loose fat in your body.
Plus you need to do some work on a few things such as horse stance and work on your kicks and strikes. It is also good to do forms since forms warm up the body and also stretch and coordinate the movements and balance. This is part of athletics.
You don't need to lift weights. Do what i suggest and you will be well above the norm. People think you need to lift and do this crossfit or MA crap. You don't Martial arts/sciences is abt control, body control to be precise. This is where forms and stances come in. Also coordination. After 6 months of what i suggest add some plyometrics. And stretching is a wrong term and misconstrueded. you need to elongate muscles. Also proper training with a qualified instructor or coach is key. To perfect the form of the technique.
For a side note i know plenty of "athletes" who enter the school i go to and can not handle the rigors of training.
I have seen and known guys who run 6 to 8 miles daily and are in good shape, supposed athletes who can not last 5 minutes of light sparring with me let alone 2 minutes. And I am not a running machine or athlete.
Athleticism is kind of a false thing. Mainly when it comes to martial arts/sciences.