Then add in weight...
For shipping to mfg..$$ For extra labor to handle..$$ For shipping to LFS..$$
Then factor in your out of the "commodity" market... 75s are a dime a dozen cause the make "dozens"... 150s don't sell at the same rate so they demand a premium price (as does glass mfg for the thicker glass)
Then add in liability insurance... Bigger tanks can cause more damage...now we all know the mfg claims "no responsibility" for collateral damage in your home, but you better believe the insurance co. makes them pay a premium "in case" they get a suit...
And, don't forget...if it costs more it "costs" more...The LFS has to front the money to stock, so there is an interest factor added for money tied up...and finally...there is a higher profit margin on big ticket items than there is on comodities...
(See, I knew all those economics classes would pay off some day...
)