Mirroring image is small with HDMI projector

When I connect my Apple TV to my 47" Vizio TV and start up mirroring on my ipad 2, the image is full size and beautiful. When I take the ATV to the school where I want to use it for iPad instruction, I connect it to an Epson Powerlite 93 via HDMI. The image is tiny. Is there any trick to get it full screen, or do I need an actual HD projector?