You can convert mass measurements to volume measurements if someone gives their answer in grams or ounces. The fact is, an answer given in grams or ounces is the best kind of answer for this kind of question. That way there aren't two guessing games going on simultaneously, and we only have to guess what your flour weighs. Based on the amount of water you are using, I would guess that you are measuring your flour rather heavy. A textbook measure of flour and water would yield approximately 92.8% hydration which is far too wet. A more realistic amount of flour would be 469 g, or 3 2/3 (textbook) cups, or 63% hydration. If this is the case, you have plenty of dough to make two 13" pizzas, and I would look to your dough shaping method as a possible issue.
Do you form a very large rim when you shape your dough? When you say "13 or so inches," how accurate are you being? The difference between 13" and 14" is almost 16% in terms of surface area. That could also be the source of your problem.
EDIT: Another problem occurs when trying to determine precisely how much dough you have when you're also using a starter. I don't know what the hydration of your starter is, but if we ignore that for the sake of the question at hand, you have more than enough dough to shape 13" pizzas.