Impossible film is different than Polaroid® film. The chemistry and components used by Polaroid® are no longer available and therefore the formula had to be re-invented.
Impossible photos are different than Polaroid® photos as the resulting frame, chemistry and even the pods are thicker (more voluminous) than the old Polaroid® film sheets. Films packs only contain 8 photos because it is simply not possible to fit 10 in the cassette. This means that the frame-counter on your Polaroid® camera will always be off by 2; when 10 shots are indicated remaining, it means 8 - when 2 shots are indicated remaining, it means 0.
The size of the cassette necessarily must remain the same size as it was conceived to fit all compatible Polaroid® cameras.
Below is an exploded diagrammatic view of an Impossible film cartridge: