#1. If I had .01$ for EVERY time I heard a runner (guy or girl) claim that he cannot strength train, espcially if it involves weights, becuase he/she "bulks up" too quickly, I could probably pay off my college loans by now.
#2. If you weight train correctly, as an endurance athlete, you would see very little weight loss but also very little weight gain. Essentially, you would be replacing uncessary fat with muscle. Fat takes up 13% MORE space (on a cellular level) than muscle does. However, muscle is more dense.
#3. Hypertrophy happens at two different levels - sarcoplasmic and myofibril. The difference between the two is that as an endurance athlete, you will not be doing weight lifting at 80-90% of your 1RM (1 rep max). This is the type of lifting necessary to induce myofibrillated hypertrophy. This is the type that competitve weight lifters (doing power lifts and Olympic lifting) do. YOU would be doing exercises that demand less weight and more reps.
If you wtick with a well planned and progressive lifting routine, you are not going to gain weight that will give you a disadvantage for running. I don't care who you are or what your genetics are. I don't care if you are a mesomorph or an ectomorph. It's not physiologically true and I'll vomit if one more person whines about how fast their biceps grow and how it's hurting their 5k training. NO IT'S NOT.