I have transitioned from the weight room to the long epic mtb rides and I find myself in the back of the pack. I am wondering if this has anything to do with adding muscle to my frame and having more body mass for the lungs to handle properly?
Well, I think is not that you have low lung capacity per se.. (Although you can increase it... and should).
I beleive the problem resides on power requirements... Bigger muscles eat up more energy, is like having a V8 vs a 4cylinder. Unless you need the torque (like when lifting weights), the 4cylinder will go much farther with the same amount of gas.
One of the by-products of muscle movement is heat (actually most energy is lost here and not used for movement). The heart has to work overtime sending blood to the skin to cool it down. Also the sweat loss is a big problem. To add insult to injury, fat on top of the muscle makes things worse.
So between the energy spent on bigger muscle movements, plus obviously more weight to carry, plus more heat retained in system, you will run into problems way before other types of bikers.
There are many details I've ommitted, but you get the idea....
Now, not all hope is lost. You should train to get better, but there are ways of setting the correct pace for your body type/condition to get better performance.
I personally find that if I get to the trail 1/2hr. before my thin friends, and warmup the engine, I can keep up with them twice as long. I also take much more care in my hydration, I use Accelerade mixture as well as water... I also watch carefully no to hit the high heartbeat area (over 80% hbmax) for more than a few seconds. Trying to keep up in a tough hill and hitting this range for long, will seriously hamper total ride performance. I prefer to catchup on the downhill (with technical and momentum advantage)...
ultimately there are just some guys I cannot ride with and enyoy my day... i'm getting better though....