20% more water in a hot tub won’t likely enhance your soaking experience by 20%, but water displacement may become an issue if you plan to pack the tub full of people.

Six swimmers will barely raise the water level in small pool due to the negligible size difference proportion, but six bathers may significantly raise the water level in a small hot tub.

A 200-pound person takes up about the same amount of space as about 24 gallons.  So theoretically, five 200-pound people in a hot tub would displace about 100 gallons of water – that’s 1/4 of a 400-gallon hot tub. Raising the water level 25% may cause it to overflow.  Then when everyone gets out, the water table may drop below the skimmer intake, starving the pumps.

If you plan to fill your hot tub with large people, you might want to choose a hot tub with more gallons, not only to make it more comfortable & spacious, but because of water displacement as explained above.  

As a Canadian, I used imperial instead of metric references on this website.  As a Canadian, I feel I should apologize.  As penance, if you bring this printed paragraph into the store, you get USD equivalent value on anything in the store, up to $50 CAD (e.g., if the USD/CAD exchange rate is 1.36, you’d get 36 cents extra per dollar).