Ok well first of all we have to think about why an ostrich has such large eyes… and for that we have to think about the ostrich in it’s normal environment. As the largest bird it could look like a very tasty meal for predators so the ostrich has a small head on a long neck so it can take in a large view of the surroundings just incase something is trying to sneak up on it. If it seems something then it will run away as fast as it can (up to 60mph!). To help it see as much as it can ostriches have evolved the largest eye of any land mammal… 5cm in diameter (which is even bigger than an elephants eye!).
Then we have to think why is the ostrich brain the size it is… well on a simple level an ostriches brain is as big as it needs to be. This sounds silly but in evolution it must have been more successful to have a bigger eye than a bigger brain, therefore all the ostriches which had slightly larger eyes (therefore better eyesight) must have been better at not being eaten, so could survive to pass on their genes to their babies. The size of a skull decides how large a brain can be, as you have to think of the skull being a rigid box surrounding the brain, so if you give up more space for a big eye in your skull then there is less space for the brain.
Also for what an ostrich needs in the way of brain function their small brains do just fine. Just don’t ask an ostrich to help you with your maths homework!!