That kinda proves it is smaller than average Voyagers.
The Classics Voyager Optimus was noted from the very beginning by Hasbro designers that it was too small for a Voyager, but too big for a Deluxe (to make a profit from that size-class price point), so they tried to bulk it up as much as possible with the two guns. (I guess the robot mode had to be in scale with Megatron and other Deluxes, which left the truck and total mass a bit small for Voyager)
And DOTM Megatron has a quarter of its bulk in the mechtech weapon, making the robot a lot smaller than previous movie Voyager Megatrons (which was likely to be a strategic gimmick to reduce the size of the actual figures from then on by including a large weapon in that year to give the total "product" the same size, but then following years the toy remains the same without a large weapon).
2007 TF1 VOY Megatron - 226gms
2009 TF2 VOY Megatron - 209gms
2011 TF3 VOY Megatron - 120gms (plus 45gms for the mechtech gun & rubber cloak)
Even including the accessories, the third movie toy was still smaller by 40-60gms (about 20%) on the first two.
http://www.toycollectors.com.au/blog/b791.jpg