Smaller paychecks at the beginning of the year vs the end?

Might be a stupid question: For the the US folks. Why are my paycheck always smaller at the beginning of the year than the last 2-3 months of the year? Been working for 10+ years. I constantly see 300-500 difference between my January pay check vs the December ones. Even tho I suppose to be making the same.

Adding context: 250k annual income. maxing out 401k which I’d divided evenly bi-monthly.