How can I be more aggressive in martial arts (Brazilian jiu jitsu)?

Whenever I'm grappling with someone else, my opponent is usually one of these two guys who weigh just a little more (around 5 pounds more) than I do and they always seem to use more force than technique so I always get pouched around which makes it harder for me to do a submission. So my question is how can I get more aggressive in Brazilian jiu jitsu because I don't want to grapple against the younger boys (who are about 12 while I'm 16).