I ended up figuring this out by doing it within Bitcoin Core itself, instead of the OS X Terminal.
These are the 3 steps I took:
1. I pulled down from the "Help" menu to "Debug Window", then clicked on the "Console" tab.
2. I typed in:
walletpassphrase "my password" 300
to unlock my wallet for 300 seconds.
3. I typed in:
keypoolrefill 10000
to increase the size of my keypool to 10000 addresses. It took a few minutes for it to complete the process.
Afterwards, to verify:
- I checked the size of my wallet.dat file in the OS X Finder, which had now increased to 4.4 MB.
- In Bitcoin Core's Debug Window, I typed in:
getwalletinfo
which confirmed my keypool size as 10001 addresses. (
Where does it get that extra 1 address from, I wonder?)
Thanks for everyone's help!

It seems to go 1 over whenever it has to create new keys. I have noticed the same several times, but I dont know why. My assumption is that it expects a change address to be used with the next transaction and thus creates one more.