Just save it as an 8-bit PNG if you want apples-to-apples comparison.
I use Paint Shop Pro and GIMP. In the former, you need to go through the properties section to change it to 8bit, or change any of the other settings. GIMP doesn't even give you this option (though it does have other settings you can alter). MS Paint is even worse, there's no way to change any settings.
When both formats were used with 8bit colour, PNG was smaller by 5%. Not worth the extra effort, IMO.
Skax459: If you're going to use Dev-C++, you might should follow Dustin's advice and use the only useful feature Dev-C++ has, the ability to download and install libraries for you. This way, it will install the correct version of Allegro for you.
To do this, go to the Tools menu and select Check for Updates/Packages. Select devpaks.org as the server, and it will generate a list of latest packages. Just find Allegro and it will automatically be downloaded and installed for you.