Compare the unit cost of travel packs against single-serve sizes.
Package sizes are only comparable once both are normalised to the same volume, and 100 ml is convenient because it matches cabin-baggage limits. Travel packs almost always carry a large premium per 100 ml, and expressing it as a percentage plus a cash amount per pack shows whether decanting a full-size product into a reusable bottle is worth the effort.
Cost per 100 ml
Cost per 100 ml = price / volume in ml x 100; premium = (travel per 100 ml - full size per 100 ml) / full size per 100 ml x 100
Packaging and handling dominate the cost of a small container, and travel sizes sell on convenience at the point of need rather than on price.
Yes, provided each container is within the cabin limit — commonly 100 ml — and fits the required transparent bag. That is the cheapest way to travel light.
For products you rarely use, where a full-size bottle would expire first, and for liquids you would rather not risk decanting.