The thing about needing to start solids because of weight doesn't really make that much sense when you think about it... What do we start with? Cereal or maybe fruit and veggies. Not that many calories and quickly digested - much faster than breastmilk which is mostly protein and fat. You're supposed to wait until after 6 months to start protein solids like meat (I believe the rationale is that it can put too much stress on their kidneys too young to have such concentrated sources of protein), and protein is the type of food that is digested slower anyway.
I think a lot of people forget that for the first few months, solids are really just for practice and to get used to different textures and tastes - not for their caloric content or even for nutrition. The only thing you really need to worry about in the first year (if you're breastfeeding) is iron, and that is only a concern after about 9 months, if they're not eating cereal or meat.
It's very possible to wait until 6 months with a big baby. My friend's baby was over 20 lbs at 6 months (my dd isn't there yet and she's a year!) and she had no problem waiting - she was only breastfeeding - no formula and he was sleeping through the night. So was my nephew.
All this just to reassure you that if you're determined to wait, then weight doesn't have to be a factor!
