There are a couple of reasons why I think you might see a 1250 watt amp. If you look at those subs that have a relatively small driver in a small box like the
True Subwoofer by Sunfire, you need to make up for the relative size of the system by increasing the wattage of the amp. This sub uses a 2700watt amp.
Another reason why you might need a bigger amp is the relative sensitivity of the sub. If one sub has a sensitivity of say, 86, it would need an amp with a wattage of 1250 to be as loud as a sub with a sensitivity of 90 and a 525watt amp (you get about ~4db more with 1250amp over the 525watt amp).
I'd expect someone smarter than I in these things to chime in, but those are two reasons I came up with off the top of my head.
JCD