Well, it's more like the cheek, but it doesn't sound nearly as cool when it's accurate.
Requested by User:Crabwhale on Discord
Megatron is 387 Pixels, and Unicron's head is 675 pixels in height and 276 Pixels in radius. That makes for a discrepancy of 1.74418604651 and 0.71317829457, which, times Movie Megatron's offical height of 10.668 meters (Note that Transformers Prime kinda bases itself on Bayformers) makes for 18.6069767442 and 7.60818604647 meters.
pi times 18.6069767442 times 7.60818604647^2 is 3383.66949169 Cubic Meters, or for our purposes, 3383669491.69 Cubic Centimeters
Now, it was split into fairly large pieces, so times 8 for frag makes
27069355933.5 Joules, 6.46973134166 Tons of TNT, high enough into Large Building level to avoid RWBY matches