yeah, I have the same issue. I'm the lightest fighter for my age group, and it was a big disadvantage to me the first few weeks of training.
To help myself I learned how to distribute my weight and move quickly to avoid pins, armbars, and triangles.
Knowing what you're doing is also a big advantage. Learn fast, and practice what you learn as much as possible.
One last thing--> I went agianst all boys in my submission matches, and while I lost them both, I lost because I got worn out, NOT becuase of my weight, or lack thereof. When you get in a fight, it's a lot different than training. Adrenaline plays a HUGE factor in the outcome.
Hope this helps,