Point Clouds are typically written with an Origin and the Base Coordinates and then Partial Coordinates for all the points for exactly this reason - the files will be huge if you have to write full coordinates with everything in mm. We Support most of the industry standard Point Cloud Formats - are they trying to use the generated file just as Point Data? Would they accept a Grid of data at lower intervals than the full point cloud. You could also ask if you could shift the Point Cloud to bring the centroid down to 0,0 and then give them the shift Data to get it back to Full Coordinates and that would reduce it somewhat - because the Coordinates would be smaller. PTS is a Point Cloud Format and I guess you can scale the Point Cloud by 1000 and then write it out as a PTS file and tell them the units are mm etc.
I cannot do anything about the file size - PTS just writes the values out in entireity - other formats like LAS are compressed so find out what formats they can read.
Alan