stretching it out?

how can I increase my batch size before I bottle my brew?

Brew a bigger batch of beer, next time.

Just bottle this as is, and enjoy it. Presumably it's not going to be the only batch of beer you ever brew. So chalk up your losses on this batch and move along, and prevent whatever caused you to have less that you planned, on subsequent batches.

What would you think you'd do? Add water, and dillute what you have and possibly infect and or oxydize it? Just leave it be and enjoy it.

Can't do anything at bottling to increase batch size.

I'm curious as to what you thought could be done at this stage to make the batch larger...