Good advice from both Mo and Brett. When you exercise you increase the insulin sensitivity and if you have too much on board, it shunts the blood glucose into the muscles more rapidly than you can replace it from other sources. A functioning pancreas would cut back it's insulin production and increase the counter regualtory hormones to compensate. If you've just bolused after eating you don't have that opportunity so you are burning sugars not fats, and won't be losing weight.
So you have two options really, either exercise before eating, or modify your bolus dose. The ADA in Handbook of exercise in diabetes give a guide of up to 50%, Think Like a Pancreas gives a table with a few ranges based on duration and effort, with a starting point for 40-60 minutes of moderate intensity giving a 0.67 multiplier i.e. reduce by 1/3.
You will need to experiment yourself to find what works for you. I've now found for example, that if I go for a long run after breakfast, if I reduce basal by 50% and bolus to 1 unit that morning, I can run happily with about 45g carbs per hour and I'll be testing about every 45 minutes just to keep an eye on things.
For shorter stuff, I tend to do that pre-meal, and have a 10-15g snack beforehand if I'm lower than 5.5, adn maybe reduce my post meal bolus as well.
Basically, if you want to avoid post-exercise hypos and the need to eat extra carbs you WILL need to reduce your mealtime bolus if you exercise after meal. You'll need to determine how much by trial and error, but it should stop you needing to go so high after dinner to manage the exercise.