The Cake twins, if you’ll be so kind to recall, were aged up enough in JG to share a grade level with the CMC and, as it were, Rumble. Pound and Rumble bonded when they were pitted against each other for sparring in the boxing ring. Mutual admiration of the sport lead to mutual admiration of each other. :)  
Pounderville Cake is a latent pegasus (his parents may be earth ponies but his maternal grandfather shared Pound’s breed), so he has smaller wings than usual. They still aren’t as small as most latent wings (think Scootaloo or Bee), and he is strong enough to stay airborne for short periods. Pound is naturally tall and bony like his dad, so he took up boxing to get some meat on his skinny frame. Rumble, meanwhile, is known as a “muscle wing”: he pumps iron with disregard to the threat of getting too bulky to take off. Nopony need worry about him, as he flies just fine. Regardless, he would rather be in the boxing ring (or better yet in bae’s arms) than in the sky any day.
 
Also, as of my writing this the canon colts don ’t have marks. I made these up.