Hi!
I am new to this forum (was searching for some infos) and found your question. I have a solution. My background is from computer graphics and 3D design.
I have really large area (in real life scale) and huge mesh and was having problems with texture in the beggining. I guess you have only one mesh in Photoscan and you just export texture?
There are two things you can do:
1. Make larger texture. When you export texture default size is 4k (4096 in settings). Try to bring this value up - 16k (16384). This will make larger file -> larger texture. For better understanding open both textures in Photoshop and compare them. You will see difference. Keep in mind that this will slower your PC since it has to load huge texture.
2. More efficient approach to this problem is that you divide your mesh into multiple parts (more chunks if you want) and make textures (4k, 8k, 12k, 16k if you want) for each part.
Just try those 2 things and you will get better results
it just need a bit of understanding about textures and CG
have a nice day.