If Sayallafter is correct, and I don't doubt it, then carrying 85 passengers and bags in a 737-200 taking somewhat longer than a 737-300 on the route (and burning more fuel per hour) totally defies logical economical sense.
I understand a -300 can take 12 tonnes of passengers/bags/freight and it appears the -200 can take around 9 tonnes. Thus 3 trips in a -300 equates to 4 trips in a -200. Given the higher fuel burn of the latter, the entire exercise of shifting 36 tonnes from Brisbane to Honiara could probably be done in the -300 for 50% of the fuel the -200 uses overall. Now wouldn't that be a saving to Solomon Airlines - paying for 3 trips in lieu of 4 for the same result.