That's completely normal. There are various reasons for it, including:

1. Air density - cold air is denser than hot air
2. The fact that your tires will likely be flatter in the cold - take a full balloon outside in the cold and you'll see what I mean
3. The amount of clothing you're wearing - the more you wear the more weight you're carrying around with you and the more constricting it is
4. The fact that cold can cause parts of the bicycle to freeze up or not run as smoothly, more friction