Still strange, They wouldn't make it 200MB if the contents weren't bigger than 100MB, and 100MB for Skype is still ridiculous
I have Skype installed and it's 37.8MB. I'm not sure why yours would be 100MB.
Just about every app you download as a disk image will have a size considerably larger than the application itself. Although you can create a disk image that is the size of a directory, you can also create a fixed size and it appears that most people do this. I'll post some examples of this later.
EDIT: As promised, here's an update. I created a 100MB disk image in Disk Utility and, as expected, I was given a disk image with 100MB of space (102.4MB to be precise). Creating one from the command line and telling it to make one out of a directory (thus, no fixed size), it created one with 66.3MB of space. I have no idea where it came up with that value but it decided that 66.3MB was the right size.
In essence, this highlights how the space available on the disk image has absolutely nothing to do with the size of the application. The number your reading is the space available on the disk image which, as shown here, can vary tremendously for the same contents.
In short, if there is shoddy coding here, it won't be reflected in the space available to a disk image.
Visual evidence: