Why do our electrical utilities use transformers way over their rated KVA?
Distribution transformers have large masses of metal and (often) oil in play, they can run at large overloads for a LONG time before the heat builds to a problematic level. 

Because of this they are often sized for the RMS load integrated over a period of several hours, which allows then to be far smaller then you would expect from a maximum rated breaker calculation. 

Remember also that IIRC the NEC specifies that loads should not be more then 80% of breaker rating, so that 600A service is really 480A design maximum, and how often are you maxing that out for more then a few minutes at a time? 

Incidentally if the breaker is 600A in each leg of a split phase service then the power is 600A * 240V = 144kW, but I would not be at all surprised to see a transformer somewhat smaller then that used, as I say integration times are a thing in this game, 50% overload during peak hours is a pretty standard place to be, but the load mostly disappears over night so everything gets to cool. 

I have seen an amusing problem when a potter took on a light industrial building and installed two rather large three phase kilns (Only one could be run at a time, but once the first one starts cooling, spark up the second one), seems the local transformer did not appreciate back to back 12 hour raku firings.

I am somewhat surprised that 600A single (split) phase service is even a thing, over here if I wanted 144kW service it would be three phase.