Onions need a high-nitrogen fertilizer in which to increase bulb size. If you want to get your onions to grow bigger, nitrogen is key. Tomato fertilizers are more balanced in their NPK ratio, for example the Expert Gardener Tomato & Vegetable Garden Plant Food Fertilizer from Walmart has an NPK ratio of 12-10-5. They can be beneficial to onions as both nitrogen and phosphorus are important nutrients for the plants. A tomato food will offer a good level of nitrogen to grow the plants and phosphorus to help the bulb grow, however, if you want the biggest onions then focus on a predominantly straight nitrogen fertilizer.