What can I do to make beaten egg puffier and fluffier?
Yes, a few suggestions:

(1) You are most likely cooking it at too high a temperature. Scrambled eggs will come out maximally tender if you do them over very low heat, stirring almost constantly. High temperatures cause the egg proteins to knot up and become tough. For 4 eggs, figure on at least ten minutes of slow cooking.

(2) You can incorporate more air by whisking more vigorously, or if you want them really fluffy, by running them in a blender before cooking.

(3) To increase tenderness, you can also add an extra egg yolk, or more butter.