Cargo Weight Limits

Hi, I am going through the process of deciding on the Rotax 915is versus the Titan engine and am confused about the weight limit of the cargo area. The cargo area limit for the Rotax 915is (120lbs) and the Titan (180lbs) does not seem to be justified by the gross weight limit. Is this just a balance issue with the additional weight of the Titan?

Is there a sample Weight and Balance form for each engine build that is available? Is this really expected to be a limitation?

Thanks!

Rob