At the moment, there is no limit to how much a vehicle can drop off at a Depot in a Vehicle Routing Problem.
I have a waste pickup truck that starts/returns at Depot "A". Depot "A" is a truck depot - it is not an unloading site. It is the start and end point for the route only.
I have established a separate Route Renewal site and associated it with the truck - this is the only location the waste can be unloaded at.
However, after the last visit to the Route Renewal site, the VRP solver continues to pick up waste on its way back to Depot "A". As mentioned, I don't this because waste cannot be unloaded at Depot "A"
ESRI explains this here
From: ArcGIS Desktop Help 9.3 - Solving a vehicle routing problem:
"For a route that has renewal visits, only the pickup orders after the last renewal visit are unloaded at the end depot"
So, there is currently no parameter such as [DepotCapacity] for a Vehicle Routing Problem. This is a shame.
If there was, I could set [DepotCapacity] = 0, to prevent any pickups on the final return to Depot "A".