The world is full of creatures whose sole purpose seems to be annoying* the local megafauna (including yours truly). Mosquitoes, horseflies, botflies, ticks, you name it.
Many of these are quite large, for invertebrates anyway, and as I understand it at least some specialize in feeding on large animals.
How large do these animals need to be, though? Is there a size limit? Would a horsefly bite a mouse, and would the mouse survive it?
In areas where humans have eradicated ungulates and the larger carnivorans, do these parasites die off, switch to smaller prey, or move to more populated areas?

* - I know annoyance isn't their primary objective, but it does sometimes feel that way.

as you rightly say this is all about optimal parasitism. As a general rule parasites don't kill their hosts though make them unwell. Yes occasionally death can occur but it is a slow process which gives the parasite maximum time to feed off the host.

From that you can see that a horse fly drinking a substantial portion of a mouse circulating volume in one go would not be sensible for the horse fly - it would stop being a parasite and become a predator! In contrast ticks etc do feed off mice just as with other animals - they are much smaller. To quote the old nursery rhyme “…and little fleas have lesser fleas, and so on ad infinitum”.

I doubt if anyone has quantified a size ratio of parasite to host or compared say volume of blood extracted to total circulation of the host but one would expect it to be well under 10%.

I'll be interested to see what others on the site think.