IMO, no, you never need 8oz bottles for a breastfed baby. One of the reasons I was so successful pumping and keeping up a large supple while working, I believe, is that I always kept his bottles 3.5/4oz, even up to 15 months when I stopped pumping. Formula needs increase as a baby grows, but breastmilk stays similar in quantity and changes in quality to make up for what the baby needs at that time. So I was also mindful that I wasn't constantly using freezer milk and that I was giving him fresh current milk most of the time.
I left 3 bottles a day home with him (each 3.5-4oz) and most days he didn't need the 3rd unless I was going to be a little later, so I'd roll it over into the next days. I was pumping more than this, so I always had some in the freezer and donated once a month to a local mom who adopted a preemie when I had well over my emergency supply. By giving less during the day they nurse so much more when you're together and your supply gets a boost. That being said, we nursed a lot during the night that first year to keep up my supply and that was a bit tiring. ;-) If you try to give a lot during the day then they aren't as hungry at night and your supply sinks.
Sorry if you know all this already and my long winded answer wasn't needed, but I really do believe some Moms have trouble pumping because they are giving too much during the day and mild reverse cycling makes things much easier.