Mesh.CreateFromBrep(brep) returns too many vertices

[chanley (Chris Hanley)]

What are the units of your drawing? If if is millimeters/inches, maybe try setting it to meters/feet and scale the drawing? It sounds like a “too far from origin” thing.
more info here:

Moving an edge on a geometry, far from origo

As was said already, you can either move closer to the origin or use meters. What is going on here is that the math in Rhino 5 uses double-precision accuracy for its operations on geometry but the display mesh is only single-precision. This has been changed for Rhino 6. If you search a bit here on the forum you will find a lot of threads on this. Just one example: