Plaetoniu

It isn't a cylinder of pure fuel floating in space. The ratio of structural mass to actual fuel decreases. Consider the difference between 3x 330ml coke cans stacked on top of one another vs a single 1ltr bottle of coke. Which has more structural material? That said, it doesn't justify anything like the first example of a 100% increase in fuel-to-mass ratio.

'At the same time, you have 3x the fuel capacity of the normal liquid tank in your super large size tank for 2.4x the mass.' Not necessarily unrealistic - larger tanks are more mass efficient than multiple small tanks. Not sure how this compares to RL, but the improvement there is not huge. The others I agree are probably too good.