like atom said jiu jitsu is alot of leverage and technique to over power the stronger biger opponent.
i think Matt Hughes proved against Royce that being 100% technically proficent in BJJ is no guarantee that you can defeat a stronger, less technical fighter. Jeff Monson is another example of an enormously strong fight that can use power to overcome technique.
Go pickup "body for life" my Bill Philips. A great starting point so can grasp some basic knowledge on nutrition and lifting. (So you dont end up like one guy on this forum who asked why he was tired when he drinks so many energy drinks... lmao!)
ive only be going for a few months as well and im even lighter (and im pretty tall) so I hear you. Ive found that just doing a lot of ab/core work has been pretty helpful, but upper body strength has been a big problem for me.
A great tip I got a long time ago was to superset squats/leg presses with upper body work, i.e. bench, curls, butterflies, etc. Why, you ask. Well, the reason is because that doing lower body work causes your body to produce more testosterone, as that region of the body has the largest muscles in the entire body, which in turn will help to build your upper body strength. Try it out for a couple months & see if it starts building your strength. Good luck to you.
get a stability ball and focus on your abs and lower back. that alone will help you by, as mentioned above, strengthening your core. alot of BJJ requires movements at the waist, such as turning the hips to set up subs and sweeps. Jeff Monson uses a Stability ball and he says its helped him immensely.
Push-ups with feet resting on a stability ball... the first few weeks you feel your body swaying all over the place, and that is weakness in the core. When I started doing them it gave me pain in places I hadn't felt pain before, and my core is extremely well developed from practising Ashtanga yoga.