To understand how small girth differences gives so much difference in feeling of massiveness, the added weight is an important factor IMO. To simulate this, maybe you can fill a condom or cylindrical balloon with water. That would give a better idea than using paper rolls.
length * (girth)² 4 * pi
Volume is proportional to length Volume is proportial to girth squared
Therefore a 20% increase in girth (5” to 6”) is equivalent to a 44% increase in length (6” to 8.6”), in terms of volume added.
[20% increase = factor of 1.2 ; 1.2² = 1.44; factor of 1.44 = 44% increase]
When you consider that people’s BPEL is almost invariably greater than their Girth, you can see how a seemingly small increase in girth makes such a massive impact on apparent size.