I found the Hiwin blocks seemed to move easier with more weight applied so maybe mass and preload of the bearings work together ?
When bearings have a high preload, they will have a "stiction" force that needs to be overcome before they move. Adding weight makes this "stiction" appear less. Unless you have high preload bearings on a lightweight machine, this issue is more "perceived" than actual.

While it's certainly possible to build a gantry that's too heavy, it's rather unlikely. Especially with a servo powered machine. If you keep the gantry weight under 125-150Kg, you should be fine.