I'm fairly big(6'3 220 when i started) and I just thought it would be something fun to do that would get me in shape. Which it did, while also turning out to be a huge lifestyle change and the best thing I've ever done for myself without question. I honestly feel like bjj was a huge part of my life that was missing and I didn't even know it. Once I gave it a try I knew right away that it would be something I would do for the rest of my life since it aligned with notions and feelings about life I already had, and not only that it grew and stretched these ideas. Everyday bjj continues to open my mind and gives me countless analogies for complex life choices. It is also a great social outlet to meet new and interesting people who aren't drunk.
Another reason is because bjj is the first thing i've ever done that I haven't got bored with. In fact, the farther I get the more I realize how far away I am from my goals and this always leaves me wanting. I am enjoying the journey and excited to know that, even if i do reach the top, I will always be able to learn more.
TBH, I could care less if I know how to fight because I'm not a dick and I live in a nice neighborhood, but bjj has changed my life in all the other (more important) areas and no doubt will continue to do so.
So, I may be bigger, but that is why I train.