No, with most drugs the effective dosing is on a mg/kg bodyweight basis. In the world of anesthesia (putting people under for surgery), this presents a problem because lets say you needed to maintain an administration of 5mg/kg per minute of whatever gas is being administered to keep a person sedated..100kg person, 500mg....140kg person, 700mg. However this is where toxicology comes in because all the drug safety data would have been evaluated on normal BMI people and now you're looking at a massively decreased margin between an effective dose and a lethal dose just to be able operate on a fatbody. Now when it comes to birth control, these are hormonal drugs. Fat tissue is an active organ from a hormone perspective, it releases leptin which is a signaller of energy availability and releases the aromatase enzyme which primarily converts testosterone to oestrogen. Think the jury is still out on whether obesity/high BF% has a causal link to PCOS, or if PCOS is something you draw the short straw on at a genetic level and it's exacerbated by obesity - but with PCOS what you tend to see are elevated androgens (T), which give rise to basically angry fat ugly women with mustaches. And it's not to say that all women with PCOS get fat, but pretty much every fat aggrocunt you see online claims to have PCOS. So you have your little 'dump the uterine wall' pill, competing with the hormone mess that is your typical obese woman. On top of that, these drugs are reasonably old, if you consider the average woman in the US today weighs as much as the average man in the 60s (so they're about 30lbs heavier), and most of these drugs were developed in the 40s-80s around the peak of the sexual revolution, there's no money in trying to make it more effective just for fat people because it'd probably become harmful to normal people.