The glass of an electric light bulb is not much thicker than the paper of this page, yet it with stands a strong grip when you push it into a light fitting. The explanation for this lies mainly in the in the bulb’s shape, which exploits the eggshell principle.
Aeons ago, nature found a solution to the problem of preventing eggs from being crushed by the weight of the hen bird as she sat on the nest to incubate them. The solution was the characteristic egg shape, which provides structural strength, to withstand all-round pressure even with a thin shell. (If the shell were too thick, the chick inside would not be able to peck its way out.)
Light bulbs (and eggs) have a rounded profile over the whole surface. When you grip a bulb, the force you apply is transmitted in all directions away from the point of contact by the curve of the glass.
This results in the force being distributed over a wide area, and no excessive stress being set up at any one point.
Picture Credit : Google