A small memory leak has been found in 3.1.10 and 3.0.7. Motion grows round 4 kbytes when you connect to the webcam mjpeg stream. A patch is available for download.
Download the patch from the patch area at SourceForge Motion Site.
The direct link to the file
The patch is made for 3.1.10 but also works on 3.0.7. It gives a small warning about an offset when you patch but that is OK.