Judging from the ones I've been in lately the Low ones contain less ships, but the overall spread of types seems to be the same. One Low Intensity Zone had several Pythons and Anacondas in it, which ripped my shields off every time I got caught in the middle of them since they all seemed to want to focus fire on me. Other Lows have had more lighter ships in them but nearly all have at least a couple of heavies.
On the other hand the one High Intensity Zone I jumped into had so many ships of all types in it that it visibly lowered my frame rate. Scary it was. Shan't be doing that again in a hurry.