I guess they wanted to offer new designs to entice more people to buy them? As for the sizes, I heard that it might be because oil prices jumped up really high earlier this year (around the time these toys were being manufactured) - due to the emerging civil unrest in places like Egypt, Libya etc.

I personally don't find the size difference that bad though... the DOTM Voyagers still feel "compatible" with the Voyagers from the previous movie lines to me. It's not as bad as say, the ridiculous size differences between some Beast Machines Maximals