The SRV looks like it folds in on itself when stowed in the hangar - which would not leave space for the two cargo canisters.
It only folds the wheels and gun turret in. The cargo canisters are (presumably) in the rear hold of the SRV considering they don't show up on the outside.
As such, +1 from me to add the SRV cargo capacity to the SRV hanger. But it means the cannisters stay in the SRV, so the next time you deploy it, it would be full.
Basically I think it should work as follows:
* Any cargo scooped by the SRV stays in the SRV Cargo Hold even when boarding the ship. It counts as part of the Ship Cargo Manifest for purposes of buying/selling at stations, although it does NOT count towards the Ship Cargo Capacity when scooping or jettisoning in space. (At a station there's room to take the cargo out of the SRV. While flying in space there isn't as the SRV is flat-packed inside its hangar).
* An empty SRV Hangar Slot counts as standard cargo capacity (ie, can scoop/jettison even when in space), and has a capacity of 4 (6?) canisters. So an explorer could choose to fit a larger SRV Hangar but not fill it with SRVs in order to gain some cargo space.
* You can only transfer cargo between the Ship cargo bays and the SRV cargo bays when in the SRV or when docked at a station.
* When you transfer cargo from SRV to Ship it only works if you either have an empty SRV Hangar Slot (a given, since you're in an SRV), or space in a cargo rack. Cargo racks are filled first.
* When boarding a ship, if any cargo is stored in the SRV Hangar Slot, it is moved into the SRV Cargo Bay. You cannot board if SRV Hangar Slot cargo + SRV Cargo Bay > SRV Cargo Bay Capacity (2).
* Cargo in the SRV stays in the SRV (unless transferred or sold under the above rules).