doey...try not to compare the volumes of today with yesteryear....
coz boards are wider these days they have a lot more float and stability
if ur sailing in typical melbourne seabreeze condtions (around the 20knts)sounds like your going a tad to big for ur skill level and you'll end up bouncing all over the bay.
e.g. I'm 90kg, intermediate... used an allrounder 04 jpsuperX 105l on the bay, could uphaul it if i needed too and sometimes found it tooo big on the stong seabreezes.
maybe drop down to around the 100l mark but pay more attention to the shape of the board and is characteristics rather than just the volume. And as mentioned before pop into one of the shops and they'll get u dialled into what u need.....just dont let em talk u into buying a kite