BPIAB mini vs. La Bassine: the numbers don't add up!

[tamagotchi]

I'm comparing the Birth Pool in a Box mini with the La Bassine (because we don't have space for a BPIAB regular size), and the numbers don't seem right.

BPIAB (mini)
internal dimensions: 45" x 37"
water capacity: 127-140 gallons depending on how full it is
(from http://www.waterbirth.org/mc/page.do?sitePageId=45085 )

La Bassine
internal dimensions: 53" x 41"
water capacity: approximately 100 gallons
(from http://www.yourwaterbirth.com/catalo...products_id=30 )

How can the La Bassine have larger internal dimensions and a much lower water capacity? I don't get it

[Romana]

BPIAB mini is four inches deeper than La Bassine.