1)Thicker air in the cold.
2)As the air inside your tires gets cold the pressure goes down, unless you correct it, you are riding with less tire pressure, more rolling resistance.
3)Tires and tubes are harder to bend in the cold, even more rolling resistance.
4)Grease can be a lot thicker in the cold, in the BB, the wheels, the pedal bearings, and the pawls inside your freehub or freewheel. If it gets cold enough some pawls don't go back and forth and the cogs just spin. It varies with different greases.
5)Chain lube is stiffer in the cold.
6)Winter clothing makes you heavier. Weigh your entire cold weather outfit. It could be ten lbs. an easy five.
7)With thicker clothing it is more work to bend your hips, knees and your ankles as you pedal. When is is below 20F this is a big factor for me, I have a lot of layers on.
8)Extra calories are used to keep warm in the cold no mater what you do.
9)Even thought it tastes better to many, drinking cold water requires calories to heat it up inside you.
10)It is more often windy in winter where I live. YMMV.
11) It's often harder to stay in peak condition when the weather is lousy.
12) I gain weight in the winter, YMMV.

Like just about everything in cycling, the end result is the total of many small things added together. Summer or winter.