Problem while using the sculpt tool

So I just started making a rocky mountain for a game I’m making with a friend, and I had a problem while using the sculpt tool. So far I’ve subdivided a plane into a 10,000 by 10,000 grid and then used the sculpt tool to make some basic rock shapes which I will add details to later.
Here are the rocks which don’t have any problems with them -


And then here is the issue I have observed (circled in red)


As you can see, the edges of the rocks have been stretched and they don’t look great. Since the player will be much smaller than the whole object, you might be able to see why I want to get rid of this problem. Now I have tried subdividing the whole object, and it works to a certain degree, but doing so adds so many vertices that my game engine will have trouble rendering and using the object if it is subdivided too many times.

Does anyone know how I might solve this problem, without subdividing the whole thing?
Thanks.