I only assume she decreases in mass because her entire body is converted into energy while in Morph Ball form. See Metroid Prime for proof.

As for your other Morph Ball comments, you're only partially right. You're basing all of your knowledge off of Metroid and Metroid 2. Metroid Prime tells a different set of facts in regards to her use of the Morph Ball, and she certainly moves faster in Ball form in that game. She's also got that Boost Ball ability which makes her that much more nimble.