Mass-effect – increase the medi-gel capacity

Being below weight capacity proportionally increases power recharge speed. Being above it decreases it. Carrying less guns reduces your weight (thereby increasing recharge speed). It does not affect the amount of ammo individual weapons hold (mods do).

The wiki weapons entry doesn't currently have individual weights for weapons, but likely that information will be added soon. In the meantime on the weapon equip screen as you add/remove weapons it will give you a percentage penalty/bonus to recharge rate. This can give you a base idea, but ultimately if you use a lot of powers you'll want to equip as few weapons as possible and put some points into increased carrying capacity.

A human doctor will be looking for the medi-gel formula you have in your possession following the completion of your mission on Palaven. You can find her in lobby of the Huerta Memorial Hospital on the Citadel, to your left when you walk in the door:

Outside of having completed the Palaven mission, there's nothing in particular that you need to do to trigger this mission beyond talking to her once in possession of the forumla. As Raven Dreamer notes in the comments, the doctor does have a side-conversation that would clue you into her need for the formula to begin with which you'll overheard as you approach her.