O.k. I think I know why you're getting hung up on this. let's quickly review what these attractive forces are:
H-bond: this occurs between an O, N, or F bonded to an H and another O, N, or F. It is caused by the delocalisation of the electrons on the H, increasing the frequency that the nuceus is exposed; this exposure of the positively charged nucleus allows an O, F or N which all have free electron pairs to be attracted to it. Let's give H-bond a value 10
London-Dispersive: the electrons around two atoms approaching each other repel each other, this destabilizes the electrons which expose one of the atoms nucleus to the electrons around the other atom. This however does not last long and the exposure is much, much smaller than that which occurs with hydrogen bonding. let's give L-D attraction a value of 2 relative to h-bond
Now, let's consider pentane. C-C-C-C-C as another pentane approaches the electrons around the hydrogen get displaced a little by the electrons around a hydrogen of the incoming molecule. This produces a small attraction. so if two pentanes are lined up next to each other the bonding force is a 10 because there are 5 L-D interactions. To put those two molecules in the gaseous state you have do two things: 1) break each L-D interaction and 2) give the molecules enough energy to enter the gaseous state. What determines how energy is required to send this molecule zooming around as a gas? Its molecular weight. Now let's make some more extreme simplifications. let's say that each attraction unit, H-Bond has 10 and L-D has 2, requires 1 unit of energy for it to be broken. THat takes care of the energy requirement from 1. Let's also say that for each unit of molecular weight it also takes 1 energy unit for it to be sent into a gaseous form, i.e. if you have 2 pentanes and pentane has a molecular weight of 30, 6 units per carbon(1 unit for each proton), you would need 10 energy units to break the L-D interactions and 60 energy units to send both molecules into the gaseous form. So a total of 70 energy units is required to make both pentane molecules gaseous. Let's do the same thing for 2 hexanes. 6 L-D interactions and 12 carbons give a value of 78 energy units to send the two molecules away as a gas. to recap:
energy units required to make molecules gaseous:
hexane : 78
now put the two molecules of pentane onto the thermocouple and in a special box without any other atoms as a gas and vapor pressure doesn't exist. That the only way for the molecules to become a gas is to add energy to it. The only source for the energy to get to molecules is the thermocouple. Can you speculate which delta T would be greater if you only had 2 of each molecule to turn into a gas?
edited to add some clarity...