This might help - took this photo for you to illustrate how the stem is retained in the watch:
The part laying on the bench is called the setting lever. It has a small post on it that fits into a slot in the stem that you can see. This is what keeps the stem from pulling completely out. This is a macro photo, and these parts are from an ETA 6497, so a large wrist watch or medium pocket watch movement. So these are quite large in comparison to what you would find inside something like a Speedmaster, or a smaller movement. The post is 0.78 mm in diameter to give you an idea of the actual size.
If either the post or slot in the stem is damaged, the stem can pull out of the watch more easily. In many vintage watches the post can get damaged by someone inserting the stem back into the movement, and screwing down the setting lever screw without first making sure the post is in the slot. Also, for those who like to keep original crowns that no longer seal, if you happen to get the watch wet when moisture enters this area it often leads to this:
In the above photo the post is facing down to the stem so you can't see it. Here is what the other side looks like:
Not all stems look like the one I show above, and some the portion of the stem that resists the stem pulling out is very thin. In the above example there is a long section of full diameter stem just to the left of the post, so you can't really break this off. In this example, the stem only has a thin ring of material:
I have come across watches where the ring has snapped off in one spot, so if the stem is rotated to that position, it will slide right out. So being careful with the crown is a good idea, but if the watch is kept in good condition this usually isn't a problem.
The crown itself doesn't have a whole lot to do with the actual force required to pull the stem out, as this is more a function of the setting mechanism inside the movement. The geometry of the setting parts and how they move (the leverage available) the strength of the yoke spring and of the setting lever jumper, and state of the lubrication in this area will all have more impact on the forces required that the crown will.
This is a Panerai I worked on this year, in the process of being disassembled. The movement is the same ETA 6497 in my first photo above:
When the crown is pulled out at 1, the post on the setting lever that keeps the stem in the movement is pivoted, and it moves at 2. This moves the yoke at 3, which has to compress the yoke spring - the U-shaped spring, which is quite a heavy spring in this watch. In watches with a lighter spring, this will not require as much force.
Ideally you would not really want to pull on the crown with a lot of force, and I would suggest that if pulling the crown out requires enough force that you are concerned about it pulling the stem right out of the watch, the setting area may need some lubrication or other attention.
Hope this helps.
Cheers, Al
