Civil forfeiture doesn't require the government to convict anyone, or even arrest them, for that matter. The vast majority of the money that comes from this process comes from people we're supposed to assume innocent, since they haven't been proven guilty. Due to that, we have to assume that all of the money that comes from this process without convictions is stolen from innocent people.
If the government wants to seize the assets of people it convicts, that's one thing, but taking money from people and making them pay to prove it wasn't related to crime is absolutely theft.
Like how you put it, it's that the scope of forfeitures is bigger than stolen goods. The comparison is only useful to help give a size of the problem. The critique is in 2 parts: 1, at least a significant portion of civil forfeitures are illegitimate, if not most of them and 2 this isn't a small problem but instead on the same order as stolen property.