Take your prenatals and always come to your OB's appointments to monitor you and your baby's health.
Eat green leafy veggies and fish. Avoid sweets, processed food, and other artificial flavoring (including Magic Sarap and the likes). Drink lots of sater especially now that it's hot, but avoid sugary drinks as well as caffeinated ones.
If your OB isn't worried about your baby's size and your ultrasound looks normal, there's nothing to worry about the size of your bump. It'll get bigger in time. For now you can also do light exercises to prepare your pelvic floor muscles and alleviate back pain if you have any.