Tall people with a high BMI will always have a hard time lifting relative to their weight.

Worry more about losing weight if you can. You will do a lot of running, and the extra weight will make your runs more difficult on your cardio and your knees. My goal is to lose 20 ish lbs, to make everything easier (240 lbs at 6'3"). And when I reach that goal, I'll see if I can lose some more without compromising my strength too much.

Ok, I found some of the science that explains the difficulty larger/taller people have:

taken from

http://physics.stackexchange.com/Strength

Strength goes like area. Intuitively, the cross sectional area of a muscle counts the number of muscle fibers (actually, myofibrils). Thus, S∝A∝L2. But mass goes like volume, M∝V∝L3. Therefore strength is proportional to the 2/3 power of mass,

S∝M2/3.

This equation expresses the fact that an increase in mass does not give a proportionate increase in strength. For example, adding 25% to your mass will increase your strength by about 16%, assuming your body composition and neuromuscular skills don't change appreciably.

Relative strength

In addition, we find that relative strength, strength per unit mass, goes like M−1/3,

SM∝M−1/3.

Thus, after adding 25% to your mass and getting 16% stronger, you are actually 7% weaker in terms of relative strength.

These facts are known, at least intuitively, to all athletes. In strength sports, formulas such as these are used to compare athletes across weight classes. For example the Wilks coefficient is used to ``normalize'' weight lifted. (In fact the Wilks coefficient is roughly (50/M)2/3, where M is the lifter's mass in kilograms.)

Some more interesting reading;

http://hep.ucsb.edu/courses/ph6b_99/0111299sci-scaling.html