I'm new to garlic growing, So I'm listening. I planted mine last year at the first hard frost. They didnt emerge until spring. Not sure if that is your problem, But what I did. When the frost was hard enough to knock out my bush beans I planted about a week later.

Our garlic planted in late October is always ready to pull by July 5-10 at the latest, June 25 at the earliest. I think the formation of outer corms tends to come late, after the bulbs are well along toward dormancy. In addition to earlier harvesting, try planting some of your garlic in very early spring, like late Feb., and compare your results.

I agree that August sounds a bit late. July 15th is the latest I pull anything and I also plant in October. Now, I live in PA, not OR, so I can't be certain that you are pulling too late. What is the condition of the outer leaves on the garlic head? Do you have several layers remaining? These will form the paper-like out covering after the curing is complete and if these layers are missing or damaged, then it is an indication of late harvest.

As for the size, I would say that sunlight exposure and nutrients are the two most likely culprits. Garlic needs plenty of sun; I've noticed an extreme difference in size of my garlic heads out the outside of a row versus the inside of the same row. The outside heads can be twice as big - this has lead me to spacing the garlic further apart than I was taught.

I agree mostly with what everyone else said but will chime in just to be redundant :).
I live in NW Oregon and even though the garlic harvest was the latest i've ever know it to be it still finished by the third week of July.
It sounds like you may have a few issues:
1. harvesting too late
2. watering later than you should. Watering when the garlic is finishing up will lead to heads splitting out which seems like what you're describing.
3. low fertility is usually the culprit for small garlic.

In addition to correcting the above, try growing a hardneck variety like 'Northern White', they are earlier and usually a bit more forgiving.

Sounds like you are growing Elephant Garlic....I have not seen corms on "real" garlic varieties...
I do not think hardneck varieties will grow where you live, it needs cold weather to mature....that being said, you could place the hardneck in the refer at 30 deg for a month before planting in late november..trying to immitate a cold climate.......