Aren't most doses measured by body weight? A 100 lb. woman taking four tylenol would be more affected than a 200 lb. woman. Perhaps a decent medical regulation would be factoring in whatever metrics are needed before dosing.
Proper dosing of opioids depends on a number of factors. Weight, as you mentioned, is one of them. Depending on exactly what opioid it is, you also have to consider renal function, kidney function, other drugs that might interact, and whether the patient has any genetic differences that may make them respond in a way you wouldn't expect. You also have to consider prior exposure to other opioids, as the patient may have developed cross-tolerance. Then too you have to look at the kind of social support they have, and whether they have any risk factors for adverse events. You probably want to be a little more conservative with the frail 80 year old patient who lives alone because a fall could be devastating.
Bottom line, there are so many factors in play that many prescribers pick a conservative dose and titrate upward until the desired therapeutic effect is seen.