For a first order approximation, the line frequency determines (for a given size core and number of turns) the max voltage it can handle. A 50Hz transformer can take 60/50 more voltage at 60Hz. Conversely, a 60Hz transformer can only take 50/60 as much voltage at 50Hz. Those combo 120/240V 50/60Hz transforemrs with the pair of primary windings are in reality 50Hz transformers. They can handle, at 60Hz, 6/5 more voltage, 144V or 288V. Not that you're likely to find that except somewheres in the 3rd world. Back 80 or so years ago some parts of the USA had 25Hz powerlines, and the occasional old radio set came with power transformers (big ones!) for 25Hz-50Hz-60Hz. That 25Hz 120V transformer can easily accept 240V 60Hz no sweat. Of course all the secondaries will provide twice their rated voltages.
The smaller transformer with the higher frequency is a big reason for switching power supplies. Those run in the tens of kilohertz. Though the core materials are different than that in 25-50-60 Hz transformers.