The seat is deep enough for a person to sit on, but not for her to curl up and lay down. Same problem on the floor, it's got plenty of room for your knees, but just not a good area for her to lay down. Shes about 45 lbs, so she's not small. I also have the optional storage under the seats, so i cant fold the seats up and let her just have the whole rear.
She makes due on short trips, most of the time she actually just sits up and looks out the window, and on longer trips i put the suitcase on the floor and her pillow over the seat / suitcase and it works out fine. She has "enough" room, i just wish she had a little more. A small complaint in the grand scheme of things.
I'm not understanding how your medium sized dog doesnt fit in a TRUCK... lol
From the picture, he seems to fit just fine.