Both package groups are authored to download the files from Microsoft. Both provide full setups of Microsoft .NET 4.0 frameworks. The smaller one is a bootstrapper that then downloads just the pieces the system is missing. The larger one has all the pieces any system might need.
It is a bit surprising that WixNetFxExtension doesn't offer a non-download package group. But, since you have already peeked into the NetFx4.5.wxs
source code, it should be a simple matter to copy the attributes that you want into your own package group.
EDIT:
I should also point that a bootstrapper takes command-line arguments as described here. -layout dvd
would download packages to a folder named dvd
, which can be used for off-line installation. (Obviously, NetFx45Web wouldn't be appropriate for that.)