The oft-repeated saying that a captain should be heavier than their stoker is untrue. I've captained with stokers twice my weight. When my wife allows me stoke for her (not very often, unfortunately), her stoker outweighs her by 55 pounds.
While weight differential is not critical, here are some tangential issues that explain the roots of this myth:
Captains undoubtedly need more upper-torso strength than solo riders. On average, females are approximately 25% lighter and have approximately 50% less upper torso strength than males. Because inordinate upper torso strength is NOT required, a woman who is already a swimmer or mountain bike rider will be able to comfortably captain a tandem.
When are heavier stokers a problem? A heavier stoker can, if scared, destabilize a tandem to a greater degree than a lighter stoker. Because I purposefully scare prospective tandem captains when I put them on the back of a tandem for their initial tandem experience, I've learned to be a bit more careful when scaring a trainee who outweighs me.
The biggest problem is the stoker who believes they must "help" the captain steer. A heavy "rear seat captain" with this unwarranted desire can make it impossible for a lighter, weaker and/or less-experienced front rider to control the bike. I should point out that although I have not yet found a female "back seat captain," I have encountered hundreds of males with sufficient ego to actually believe the bike will fall over if they don't help me by leaning. The most bull-headed of these men will "steer" the rear bars with enough force to twist my seat sideways. My sure-fire method to break a guy of this habit is to force him ride no-hands with his eyes closed.
Here is the typical route-to-failure that inspired the "stokers-must-be-lighter" myth. A woman either decides or is elected to ride the front of the tandem with her husband/boyfriend accompanying her as stoker. A minor problem she may have is lack of upper torso strength and/or bike handling skills. A further impediment is a woman's proportionally-shorter arms and torso may make it difficult for her to reach the handlebars on a tandem set up for a male. But the biggest problem most women will experience when they first captain a tandem is the loose nut on the rear seat. In most cases the guy has never stoked before and is scared. Next, especially if he's taller and can see the road in front of the front wheel, he'll play back-seat-captain and sabotage her ability to control the tandem by "helping" her to balance and steer. Finally, because society (including mothers) train young boys to exert control, many guys feel too vulnerable on the back of a tandem and consequently don't want this experiment to succeed. After a few awkward miles of shared misery the test is concluded and, instead of understanding the real problems (her skill and upper torso strength; his fear and ego) they save face by blaming their failure on a difference in weight.
It's too bad many guys are unwilling to give stoking a real try. Everyone I know who has gotten comfortable with both ends of a tandem prefers stoking--truly the most liberated form of cycling.
Gals: if you want to know what "captaining" is about, find a properly sized tandem and ride it with an experienced FEMALE stoker. (You might get a positive response to a question most men can't ask: "Excuse me, may I please borrow your tandem and stoker for a few hours?") After a few miles with an experienced stoker, you'll be ready to captain your husband/boyfriend--show him the blindfold you've prepared and tell him to behave himself.
Guys: Here are your instructions for stoking behind a wife/girlfriend who has already proven herself by captaining a tandem with someone else. First and most important, shut up--she doesn't need or want your coaching. Second, don't look at the road (instead, try relaxing by watching your feet go a round).
Which team member should ride up front? While weight does not make a big difference, aerodynamics does. All tandem teams are unquestionably faster and more efficient when the bigger (especially taller) rider is captaining. Further, because the stoker's handlebars are attached to the captain's sea tpost, stoker ergonomics are also improved with the taller rider in front. If you and your tandem partner have near-equal bike-handling skills and height (less than one out of one hundred teams fall into this category), your team will be ever-so-slightly faster with the most powerful cyclist in b ack where drivetrain efficiency is maybe 1-2% higher.