Many “space” fuel tanks have inflatable bladders inside to control the pressure and location of the fuel, especially in zero-g. Otherwise, the fuel could float away from your tank valve. It is possible that this tank has a fully or partially inflated bladder, making it much more buoyient.
Yeah I’m glad that some series (Pixel AFAIK) are now releasing slightly smaller phones, because the growth was getting ridiculous and highly impractical.