Before it burns out, does it work fine?
If so, it seems very unlikey to me that more than one doorbell button is pressed at one time, so I don't see why the size you've been buying is not adequate, just as it would be for a single family. Unless there is a short in a button or somewhere else and the transformer is outputting all the time. I'd disconnect the wires to it and measure the ohms between the two wires when NO one is pushing any buttons. I think it should be close to infinite. Then for kicks, I'd have someone else push each button in turn and check the ohms for each of them. Should be about the same if all the bells are the same and if the extra distance for some has little effect.
I'd take the lowest ohm reading, divide it into the volts (16 or so) to get the amperage and I'd compare that to the rate amperage of the transformer. I'd expect the transformer to be a lot higher.
What they said. I'd disconnect the 24v. transformer right now and let people yell up to the window, like normal people do. Or throw pebbles or cellphones against the window. Or call up with their cell phoen.