My guess would be not the physical size but the complexity. I think the RAN boat would be the hull and the drive train. ASC and Raytheon would then be tasked to fit the combat system, and possibly integrating with non Japanese sonars (bow and flank array). It would also be interesting to see if the Australianised Soryu class were to have a towed array sonar as well. Other changes might be a lock out chamber for SAS, etc.
These are just my guesses...
The Japanese Soryu already has tower array sonar already, so at least, the room for such equipment is already built in.
From different news so far, seems "evolved Collins" is a dead end because Kockums still holds the IP of Collins, an their German directors are more keen to sell their Type216 then the Sweedish stuff. French Scorpene option just silent without trace (I don't like that option too), Spanish S-80 is now off-favored as Navantia is struggled to make the sub float.
So right now, it seems only 3 options: Type 216, Soryu variants or joint-developed new sub with Japan still on the table.
While HDW has a proven history of exporting submarines, either build in-house or overseas, The design is an extended Type 214, which is practically a downgraded Type 212 for export. It has a fuel cell based AIP, lithium cell batteries, and VLS for SAS or land attack missiles (which is a pre-request for Sea1000). It is also modular so it is easy to install your own stuff, which Turkish has just done it on their Type 214 (everything Turkish except the hull and drive train). However, the Type 214 (which Type 216 is based on) is mired with unresolved design flaws that S. Korea complains a lot. Also, their fuel cell AIP, while silent when operating, has a low power output which can only sustain a few knots for cruising. "Cruising" at 3 knots may be good enough for German as their submarine just leave the port and then "sitting at the bottom", 3 knots is good enough for stay in position against the current. But for Australia, we need to transit 1/4 of the world to our designated patrol, so it is no way desirable.
Soryu class has its issue too. While it has the right size, it is a proven design with American based sensor and weapons, their Stirling engine AIP is more suit for Australian needs and, if build in Japan, it is way cheaper. However, its weapon system is not designed for land-attack missiles or special forces. But, at it has NATO-standard 533mm torpedo tubes, it can fit in a few tomahawk or for SAS divers to swim-out. IT doesn't have the Lithium-ion batteries as in the (still on paper) Type 216), although Soryu was planned and design to carry such battery on their 6th ship, but not implemented due to budget constrain. In order to fit-in the AIP system, the living space on board is know to be very cramp even for Japanese, so it will be a challenge to find some space for the SAS, the console for the tomahawks, and crew morale. Compare with the price Egypt paying for their Type 209 ( $460M each), $500M for a much more capable Soryu class is a fxxking good deal for Australia.
So, overall, I would rather to have a off the shelf Soryu build in Japan (with minimal changes for tomachawk and Asutralian power sockets) as one reason why Soryu is so cheap is because all R&D cost has already been paid for (by Japanese own subs). And then use the saved budget to kick-start the 2000tonne ANZAC replacement in order to shut the union up. Or, an enlargen Soryu co-developed with Japan and both country build their own submarine. The worse case will be let RAN playing that sort of "cherry picking" the features and merge it into a design and then expecting the project is still risk free and derailed just like the Super Sea Sprite...........
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BTW, I still think Australia should close down more shipyard and concentrate all defense ship-building to ASC........
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