What is the difference between under sink replacement filters and whole house replacement filters?

A href="https://www.waterfilters.net/Under-Sink-Filters.html">Under sink replacement filters fit the slimline housings. This means the diameter is slightly smaller than the standard whole house replacement water filters. For example, the Pentek C-1 whole house water filter has a slightly larger diameter than the Culligan D-10 water filter although both water filters reduce the same contaminants and are made out of the same filter media.

What is the difference between under sink systems and reverse osmosis systems?

Under Sink water filters systems do not reduce dissolved solids such as sodium (salt), nitrates, etc. The membrane in reverse osmosis systems reduce more contaminants than regular under sink systems.