Well, for one thing, you're going from one format to another -- but, more so, you're going from one codec (compression system) to another.
The H.264 codec is the one used for creating AVCHD video, although not all H.264 video is AVCHD. It can also be used as a delivery format (that's a format that you display online or play on a computer) -- which seems to be the case here. Your source is apparently from an AVCHD camcorder, although at that file size it must only be about 10 seconds long.
Though that's hard to say too. Codecs can be highly compressed or lightly compressed, so two files using the same codec and the same file format can be very different in size!
The Cinepak codec is also a delivery format, though it uses a completely different codec system as well as being for a different file format (AVI). You would neer use a file with a Cinepak codec as source material for editing. It's a highly compressed delivery format. Convert the file to a DV-AVI (an AVI using the DV codec) and it will likely be many times that size.
Along with Steve G's comments, there is a big difference in a GOP (Group of Pictures, like the MPEG H.264) and a full I-Frame structure. Plus, the Radius Cinepak CODEC is decades old, and is very ineffective. For background on GOP structure, this ARTICLE might be helpful. For general info on CODEC's, this ARTICLE will give you some background.