There is no significant difference between the two camera's you mentioned when it comes to catching detail. The D7500 should be a bit better at it, but the difference is only about 3%.

The reproduction ratio does not change. It is a lens property, so wit the same lens, it will always stay the same. The sensor of the D850 is much bigger, but fortunately it also has a lot more pixels. The amount of pixels that are used by your subject will be about the same.

Suppose you are taking a picture of a fruit fly that is about 4 mm long. At a 1:1 reproduction ratio, it will also be 4 mm long on the sensor. On the D7500 sensor it will be roughly 1030 pixels long, and on the D850 it will be about 1000 pixels long. Not a real difference. But the D7500 sensor is 5,568 pixels long, so you have 4,565 pixels before or after the fly. The D850 sensor is 8,256 pixels long, so you have 7,256 pixels before or after the fly. With both camera's you can print the fly with the same size and quality, but if you do that, the D850 will give a larger photo which shows much more of the surroundings of the fly. You are not losing anything, you just get more of the surrounding with the D850.