Assemble-yourself ones are fat because you can unscrew the case to solder wires. The iPod ones are small because they are manufactured industrially and cased, without the possibility to unmount them. That's also why repairing that kind of jacks is painful.
One solution, even if not really clean, is to use two headphones with the connector you like, cut the wires and solder them together covering carefully all the wires (make sure that they are isolated) with heat-shrinking pipe. It's a precision handcrafting work, since you have 3-4 thin wires (3 if you solder the return wire of the channels together) that have to be isolated each other, and an external casing, which is the easiest part of the job.