The free version has no limits, the source for dolphin is included. Two things that really should not even be mentioned as they are the same for free as well.
The only real difference is the Pro license turns off the powered by links, comes with an advance membership on the boonex website and lifetime version updates. A future version of dolphin is rumored to be removing the powered by links even on the free version but will charge for some of the modules that are currently included in the free version. Those that have a license before that happens will get those modules without cost. So that is part of the lifetime version updates mentioned.
The Prime is the same as pro, but you also get a Premium account on the boonex website instead of Advanced, source code for the mobile apps and boonex install service.
Not sure why they even mention this No Site Size Limit thing. No such limit exists even with the free version. That has to be a marketing thing.