I think one of the criteria is your general health and how you might tolerate the various drugs. Also, sometimes when you have more cycles the strength of each drug is reduced, so you end up having the same amount but over a longer period.
Morwenna is right, your oncologist should explain the pros and cons and why he made the decision he did.
I had a 7cm IBC with lymphnode involvment and was given 3 FEC and 3Tax pre-op, I asked my onc why I get 6 and not 8 chemos and she said she would treat me according to the European PACT trial and she would follow what leading cancer clinics do in the UK. Didn't instill a lot of confidence in me I have to say but that's it. So I don't really think it has to do with how big or aggressive etc, at least not at my local hospital!
I had 8 chemo sessions first as my tumour was quite large (45mm) and had spread to lymph nodes. They thought it was quite aggressive as it didn't show up as a lump; a cyst had formed around the cancer and my breast went from being normal size to hard as a balloon overnight. Ifit hadn't been for the cyst I don't know at what point I would have felt the lump. Don't know if this helps answer your question but good luck with the treatment.