It is likely a calibration issue for your extruder.

It works well on smaller parts because your printer is extruding less filament at slower speeds...which happens with smaller parts.

As soon as you get to larger parts, your slicer speeds things up since you have longer straight runs and larger features. The higher speed means faster extrusion, and in your case too much.

This is a good article for you to read:
http://reprap.org/wiki/Triffid_Hunte...ibration_Guide