It must be a power trade-off.
Captain Needa doesn't say, I've been on board the Millenium Falcon and I know it has too much habitable space to contain a cloaking device. He has about 2 seconds reaction time, maximum, and then he says that a ship that small couldn't have such a device.
This doesn't suggest that a cloaking device must have a minimum size, though, not really.
It only suggests that the power-generator for the device must have a minimum size, and that the Falcon is too small to contain a generator large enough to power a cloaking field, on top of the power needed to run its hyperdrive.
Hence it's a power trade-off: a generator small enough to install in the Falcon would be powerful enough to run either the hyperdrive or the cloaking field, but not both. And since the hyperdrive is essential, logic dictates that the ship can't be running a cloaking device.
We are entitled to assume, IMHO, that a generator is a generator: that there's nothing special about the generator used by a cloaking device. Any generator will do, provided it's powerful enough. But a generator of the size mounted on a Star Destroyer would not fit inside a freighter.
Darth Maul's ship might not be simply a continuity error. As Solo uses the Falcon for carrying freight, possibly even legitimate freight (carried to provide camouflage for the items he's smuggling), there must be freight bays or cargo holds within the ship: the presence of that much free space inside the ship makes the installation of larger generators impracticable, but that is a restriction which will not apply in the case of a vessel that is not designed as a freighter.
Hence, the probability is that it's a trade-off between power generation and freight capacity. And between the power requirements of the hyperdrive and a cloaking field.