I don't have an old shivermist screenshot to compare, but I have both four- and five-star versions of the bomb to compare it to.
It looks like the radius really was only slightly reduced - I can't immediately tell it's smaller by memory and eyeball. (My guess is that it had to do with Vanaduke runs, where the edge of the old shivermist haze was just big enough to keep the lug frozen while remaining relatively safe.)
http://weedwhacker.org/priv/20110706-atomizer.jpg
http://weedwhacker.org/priv/20110706-shivermist.jpg
-edit
The wiki has an old picture, looks like it was taken at one of Basil's terminals. I'll update the screenshots when I next get to one.
http://wiki.spiralknights.com/File:Shivermist_Buster_blast_radius.png
-further edit
The displayed Atomizer is at heat level 6 or 7, while the Shivermist is level 10. I don't recally if the heat level affects haze radius.
Here you go: Radii comparison using nitronome radius as baseline. I used the wiki images of nitro and shiv.