Jonny, the idea behind using the cartridges during the studio portion of the movie was the exceptional ease of handling. It allowed for much quicker absorbant changes than granules, and you don't want to be the guy tapping his cannister with the clock ticking and dozens of people waiting. On the set time is money.
This
picture of different cannister inserts on ISC's website shows from left to right the ISC Mini Meg scrubber, the standard ISC Meg scrubber, the Cis-Lunar stack, the Micropore canister and the (old 5.4 lbs) SMI PRISM basket. Gives an idea about the size.
