Surprised at that, but as I suggested earlier, folding (and feathering) props do not always follow sizing which is based on conventional fixed propellers, mainly because their blades follow a different shape and often have a smaller BAR (Blade/Area Ratio). That is why it is important to fit the size recommended by the manufacturer of the specific propeller as that will be based on their experience of what works for a particular boat/engine combination.

As a general rule, a change of 1" of pitch or diameter changes engine revs by between 200-250rpm at the shaft speed (1500rpm) on a saildrive. That is how I arrived at the figure of approx 2800 for a 18*12 (compared with a 16*11 normally fitted to a 2030). That is 2" of diameter and 1" of pitch equals up to 750rpm, so 3600-750 = 2850. However now we know that the Gori recommendation for their design of prop is a 16.5*13 which will be based on achieving 3600rpm, the revs with an 18*12 could well be 3250 (3600 - (1.5*250)).

Having said all that there is no advantage in using the larger propeller - in fact a loss as you will only be able to extract around 90% of the available power by restricting the revs. So, if you are going for a Gori, then fit their recommended size.