The 100ml bottle on the left cost $3, and the 700ml bottle on the right cost $6. If you wanted a travel size you could buy a small travel bottle for next to nothing (or reuse a clean small bottle you already have), and decant.
I often look out for well-priced large sizes of products I use a lot of. Lancôme is pretty good at frequently offering really cheap maxi-sizes of popular products, as is L’Oréal. Salon size bottles (usually 1000ml or 2000ml) of your favourite shampoos also work out much cheaper. If you find the bottles too big, decant into smaller ones.
I found this special today – the Lancôme cleanser and toner I use. The 200ml bottle costs $34. This 2 x 400ml set cost $37.
Like you I buy bigger sizes when they are better value than the small one. Recently I found my favourite spread was reduced in the smaller size so bought 3 of them for just over the price of one large which contained double the amount.