Why not? He can take a seat on your lap...

You smirk, seating yourself on the edge of the bed with your legs apart, and unzip your fly, shimmying your trousers down as your own hardening dick slips out into view. "Sure, that sounds good to me." You slowly and deliberately pat your thigh three times, signalling exactly where he should be seated during your little 'experiment'.

The imp's cock-sure expression fades slightly, and for a moment it looks as though his lips may even curl into a snarl, but he plays along, descending so that his hooves land unsteadily on the surface of the bed between your legs, and then turning around (with one last glare directed at you), before seating himself. Your erection glides up his fuzzy ass-crack and brushes against his balls, hardening as he comes to rest in quite an awkward position, lying back against your belly and chest, legs wide, with your own insistent erection sticking up between his thighs, nudging against his own.

From this position, with him on top of you, you may have put yourself at a slight disadvantage, if the goal is to gauge the relative length and impressiveness of your respective manhoods. Regardless, there isn't much more than a few centimetres of distance between the tips of your cock heads. You wiggle beneath him, pretending to find a more comfortable position, and the feeling of your warm crotch rubbing up against his balls and cock seems to elicit some kind of reaction, as the imp mewls quietly in surprise.

When you glance down again, venturing to caress the smoothness of his waist with one hand while steadying yourself with the other, you realise that the contest is clearly won! Perhaps that minor adjustment was all it took to afford your shaft a bit more room to shine- or perhaps there are other factors at play- but you've edged him out by a good couple of inches, despite the disadvantage of having him sat directly on top of you. In fact, you think to yourself, as you nestle your chin on the imp's shoulder to get a look down his front (much to his chagrin), his package looks decidedly less impressive (even for his stature) than you thought earlier. As well as length; where earlier there was no perceivably difference in thickness between the two of you, now his dick seems just a little slimmer than yours (as befits his overall size, you reckon).

Just as you're considering reaching around to have a little play with your new imp's features, he takes flight again, circling around to face you once he has regained a safe distance, arms crossed and evidently bitter. You think you may have caught him out, this time, but that doesn't mean you should let your guard down.

"Fine! I suppose that settles the outcome of that puerile little exercise. Let's strike 'penis envy' from the record, and just start afresh."