Air resistance against the bob and string and friction in the pivot make the amplitude of a simple pendulum decrease.
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It messes up the math. For large amplitude swings, the simple relation that the period of a pendulum is directly proportional to the square root of the length of the pendulum (only, assuming constant gravity) no longer holds. Specifically, the period increases with increasing amplitude.
The amplitude of a pendulum is the distance between its equilibrium point and the farthest point that it reaches during each oscillation.
The pendulum swings twice as far.
That if the original amplitude was A then it is now 2*A.
wind resistance cannot be ignored in considering a simple pendulum. The wind resistance will be proportional to a higher power of the velocity of the pendulum. A small arc of the pendulum will lessen this effect. You could demonstrate this effect for yourself. A piece of paper attached to the pendulum will add to the wind resistance, and you can measure the period both with and without the paper.