Ok so I take it todays definitions were quite large?
No they aren't and no they aren't anywhere close to 100 MB, the whole program download which includes virus definitions is only 73MB.
You still haven't shown how this 100MB comes about the full virus definitions database when unpacked on the hard disk is only 70.6MB. See image of my defs folder, it will usually have two sub folders the current and previous one. When a new incremental update is received a new sub-folder is created and the old virus definitions database and the incremental updates are merged into the new sub-folder. All of this happens locally on your system and it doesn't download the whole database.
So whatever source you are getting this from that the download was 100MB is royally screwed.
The avast virus definitions updates are incremental and are measured in KBs rather than MBs