Is a little oven less hot than a bigger oven?
It could be that your oven's temperature control is off somewhat. Ovens aren't exactly scientific instruments, so you can't be sure how accurate they are with this kind of thing. 

Get a decent oven thermometer and go by that rather than the temperature knob. You might be surprised. Mine runs about 10-15 degrees less: when I set it to 180 on the dial, it's often about 165 on the thermometer.