Re: 6 foot, 215 pounds , and struggling to do muscle ups.
The muscle-up definitely favors lighter athletes. That being said, we currently have four guys taller than 6' and heavier than 200 lbs. who can complete 30 for time in 5-10 minutes. The largest is 6'4" and 230. In addition, our fastest 30 for time is 3:35, performed by a 5'10", 185 lb.-er - not really a small guy.
Get stronger at CTB pull-ups and deep ring dips, and work your muscle-up transition progressions, as well.