There are two questions here.
1) Are larger drivers more sensitive than smaller drivers,?
A simple review of TS frequency response graphs (the sound level between about 200 and 2000 Hz) will show that larger drivers are generally more sensitive than smaller drivers, but that there is a wide range of sensitivities in a given driver size and that there is considerable overlap, with for example some 10's being more sensitive than some 15's. So this must be taken on a case by case basis.
2) will the combination of two drivers sound better when the two drivers are the same size?
Examination of the same frequency response graphs shows that while there is a general trend of deeper bass for the larger drivers, and more extended highs for the smaller drivers, there is a great deal of scatter in driver frequency range (useful response from the lows to the highs), and, just as critically, in the overall shape of the frequency response (the distribution of peaks and dips). The latter greatly impacts the tone of the driver. And in this characteristic, the drivers are all over the place. This has more to do with things like cone composition, density and profile, surround geometry, composition and density, voice coil and former mass, composition dimensions, and dust cap profile, composition, density and size. So driver size has little to do with the tone of the driver through the mids and highs. Therefore, matching cab driver size is only the most rudimentary step to tone match.
The bottom line: For matching cabs for greater volume or tone, driver size is at best a weak guide. To make any real progress in matching cabs for loudness or tone, it is essential to examine them on a case by case basis. For tone matches it is best to match identical cabs. In some cases cabs have been voiced/designed by the manufacturer to work together. For all other non-identical cabs it is best to audition them.
