You've probably seen ads for M&Ms Mega chocolate candies. According to Mars, Inc., they're 55% bigger (which matches my guess that they are about 1.5 times the size of the regular plain M&Ms). We went out and got some recently. On the left are plain M&Ms; on the right, Mega M&Ms.
What can we say about them? They're a little bigger. They have a higher ratio of chocolate to candy shell -- though since the candy shell is pretty much tasteless, these don't really taste any different. The colors are different, and for some reason, the color seems to run a little more easily. If you hold them in your hand for just a short while, you get colored stains on your palm. While the chocolate stays safely inside the candy shell, it does feel like these Mega M&Ms don't live up to the 1954 slogan "melts in your mouth, not in your hands" slogan.
All in all, they're okay. They probably work better than plain M&Ms in those M&M dispensers that people often fill with peanut M&Ms. The regular plain M&Ms are a little small for those dispensers. What we wish is that they would make the dark chocolate M&Ms available in the store, and on a more permanent basis.