I know exactly what you are talking about, but since I don't know that measurement now, I figured I would measure something I could find out, and that feeder is supposed to be 54 x 54 x 76. So maybe the 54 includes the entire width of the leg posts? Let me do that quickly...that math puts him at around 15.8 inches wide inside. Still don't think that is close, but closer than my original math (I guess I don't know dimensions of feeders and where they get the dimension from).
So, I still think there is some proximity variance in your idea, he looks to be standing a little more in front of the feeder, and a small variation in that distance is going to give or take an inch or several, and this buck may not be able to give an inch.
http://www.walmart.com/ip/Moultrie-30-Gallon-Easy-Lock-Tripod-Feeder/23569533#Specifications