I do alot of work and give a fair amount of donations to a animal rescue facility that fits thru about 400 dogs per year. Pit bulls have without question been the most likely to be aggressive out of all the dogs that file thru. We get many other aggressive dogs but the pits are the only ones that stand out.
This may be due to their strength or due to the above average likelihood of them being raised in aggressive environments. There are also nice pits but regardless I am completely against breeding them and more so, there is no logical argument to be made breed them.
I think the occurrence is somewhat more. While some smaller dogs may be aggressive and are aggressive, they also tend to learn rapidly they do not have the size to be aggressive. Thus that trait becomes less common overall.