So apparently the Optimus Prime that comes with Stratosphere is small enough to fit into Combiner Wars Devastator's hands! Also I think Optimus is Micro Machine size

I would take photos, but I'm pretty buggered at the moment so I can't be bothered.

UPDATE: Actually it's not only Devastator who can hold him. I tried it on a few figures (Titan Metroplex, MP10 Optimus, Armada Tidal Wave) and they all can hold Optimus Prime. In fact Stratosphere can also hold him.
Hope you didn't lose your lil' Optimus Prime, because a lot of bots can in fact, hold him.