Storing eggs small end down keeps eggs fresh somewhat longer.
An egg’s freshness is judged by candling, using a light to see the position of the yolk.
The nearer the yolk is to the center, the fresher it stays, because the surrounding albumin, or egg white has antibacterial properties to protect the embryo, or the yolk in an unfertilized egg.
The big end of the egg also has an air cell that could deteriorate when the weight of the contents rests on it.
Birds’ eggs have a pair of cords, called chalaza cords, that attach the yolk to the shell lining membrane at each end.
The yolk is high in fat, so it tends to rise in the egg white.
The small cord is at the big end and the big cord is at the small end. If the small end is down, it allows the bigger cord to hold the yolk better.
It would rise too easily the other way.