This is the inherent problem with robot verse alt modes. The rovot modes are so close together comparitavily in size yet the alt modes differ wildy. Look at warpath. On screen warpath is smaller than optimus in robot mode yet the bigger in alt modes. Badcube found a middle ground but the original toy put him as a tiny bot.
We will never find a common middle ground that works in both.
How does megatron turn into a gun that can be used by humans, robots and still be the biggest decepticon bad guy we want? How can a guy like astrotrain be the same size as a seeker in robot mode but than transform into a mega train and an even larger shuttle that can hold devastator with room to spare?
All we can do i find one that feels right for ourselves and make do.
Personally if you want hellfire to scale correctly with an optimus the best way would be to put him in the chug scale next to toyworld orion as your optimus. That makes him bigger than optimus and leaves the alt modes to scale. The problem there though isvthe scale of him to the rest of the group. He would be half as tall as hercules devastator and thats too big. Theres no winning.