Lets try to actually get some calcs of the verse, shall we?
Person: 18 px/1.755 meters
Colusseum height: 501 px/48.8475 meters
Colusseum height: 227 px/48.8475 meters
Culusseum width: 551 px/118.568160793 meters (59.2840803965 meters for the radius)
(Pi*59.2840803965^2)*48.8475 is 539347.151479 m^3 for the volume, or 539347151479 cm^3
Assuming that this is affecting only 10% of the volume, the volume is 53934715147.9 cm^3.
Lets account for violent fragmentation on this one.
53934715147.9*69 is 3.7214953e+12 joules/889.458723709369 tons, or Multi-City Block Level+.
There is also a pulverization end, just in case.
53934715147.9*214.35 is 1.1560906e+13 joules/2.7631228489483747 kilotons, or Small Town Level.
And what about the explosion?
Colusseum width: 422.5 px/118.568160793 meters
Screen width (What we see of the explosion): 997.5 px/279.933113352 meters (139.966556676 meters for the radius)
Knowing this is a ground explosion...
139.966556676^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2 is 220.372983394 tons, or Multi-City Block Level.