I am 12 weeks post-op with confirmed waterfall deformity, can I have larger implant to avoid full uplift?

A breast lift is almost always the best way to correct waterfall deformity, which is caused by the breast tissue sliding off of the mound created by the implant. The solution is the elevate the breast tissue, not place more stress on the tissues with a larger (and heavier) implant.

If you have this issue, even with a subglandular placement - then it will most likely need a lift to fix.A larger implant is unlikely to solve the problem.

to answer without a picture. IF you truly have a waterfall/snoopy deformity, you really need a lift, not a larger implant. If you dont want the scars, then live with it. placing a larger implant can somtimes help the deformity, but rarely fixes the problem. good luck

Implants do not lift the breast to any significant extent.  If you need a lift, then you need a lift.  

There is a limit to what sub glandular placement can achieve.  I think you probably need to discuss a mastopexy with your surgeon.