The size of the bore of an alto flute is a contentious issue. Boehm's original intention for the instrument was that it should have a bore of around 26mm, to allow for a distinct tone which was different from the C flute, even on notes of the same pitch. The effect of this wide bore is a rich low register, and fewer high harmonics in the sound, giving a unique tone to the high register, which is less strident than on the C flute.
Throughout the instrument's history, bore sizes have changed, with different makers experimenting with the design of the instrument in order to achieve a particular sound. Some composers, not understanding the unique sonority of the alto flute, have written for it expecting a similar response to the C flute, and have been disappointed with the results. Some makers, particularly in America in the 1960s and 70s, created small bore alto flutes to create the effect of a C flute with an extended low range, but in doing so they lost the richness of the low register tone. Specialist makers now offer a range of bore sizes, depending on the needs of the player. My own personal preference at present is to use a slightly smaller bore size (around 24mm) for ease of response and evenness throughout the range of the instrument. Larger bore instruments are likely to have slightly more resonance in the low register but a slower response and some loss of flexibility in the high register. Different bore sizes may also affect the response and balance of some multiphonics.