You could be taking in more water than you realize. You have to remember that most foods are going to contain a good amount of water, unless it's dehydrated or something, and even dehydrated foods contain "some" water. Almost every food you can think of is going to contain a certain amount of water: vegetables, fruits, meats, grains, ect.. In fact, all plants and animals (including us and the foods we eat) are composed of mostly water by weight. For instance, humans are around 62-65% water by weight, so you see what I'm getting at. So the more food you eat, the more water you're taking in, therefore you're going to end up urinating more.
Now this may not be a substantial amount of water, as far as how much water you're taking in by food sources, but it's a good amount nonetheless. Of course there could be other reasons why your urinary frequency has increased, but there is enough water within 1500 calories worth of food to make a noticeable difference.
