my best advice re: drawing like, truly huge tits is that your gut instinct is gonna tell you "oh, boobs are round, i gotta draw boobs round" but that is the devil speaking. Tits are round-adjacent but they are not round, and the larger they get the less round they wanna be.
a common mistake i see a lot is having the outline of the underboob go straight on up into the armpit/shoulder for huge tits, but you need to realize, as tits get bigger, that line of where the underboob wants to connect to dips lower and lower. The clean line of action into the armpit/shoulder is for kinda small or mid sized, very perky tits. when you introduce more size and more sag, that line has to be brought down farther on the ribcage and begin to flow into the muscle and fat of the back. You essentially have 3 core anchor points for the breast mass to hang from: The sternum, the shoulder/armpit and the ribcage. you can almost think of the breast mass as a really FAT/soft 3 point pyramid in a sense, where those anchor points are the base and the nipple is the peak.
also, even really big tits that DO look very round are actually much flatter than they appear when resting/hanging.
if i can find some time ill try and draw this out visually sometime soon
nothing will make tits feel weightier at ANY size than sag. perky tits are lovely and everyone's style is their own but if your GOAL is HEFT AND WEIGHT, you gotta let the tits REALLY hang.