I haven't read the justifications given, but at a wild guess:
The idea is that people trend to defaults, and current pricing schemes end up with people defaulting to the largest size. Reduce the maxium size sellable, and you reduce the default. Reduce the default and mindless calorie consumption drops for most people. Businesses spend alot of money to study defaults and mindless consumption to maximize profit. It isn't some idea that was just invented by Bloomberg. Ethical businesses try to engineer intelligent defaults and benefit consumer and business.
In principle, I get that they are trying to do a good thing. However, ON principles, I think they overstepped, majorly. Don't want you population to mindlessly consume sugary crap? How about you develop the critical thinking, self-assement, and mindfullness of your population, so they are less likely to take defaults, and more likely to default to better options in the first place.
Of course, if people actually learned to think for themselves then pretty much every politician and most businesses would be screwed and since those are bassically the same entities now...I don't have much hope for the future.