Each planet has a 'planetary body' that are pretty much always hot when they are being created. As time passes, that internal heat that causes geological activity is slowly lost. Larger planet bodies are able to keep the internal heat longer than smaller planets, and that heat is needed to keep the planets geologically active.
The only way a smaller planet can retain any heat is from tidal friction coming from another planet. The constant friction from the external body keeps that small planet from going completely cold to the core due to its size. Hope this helps!! :)