Its true that the Polaroid film packs are a different size than the standard film packs, but the film pack format was the design inspiration for the Polaroid pack.
Film packs held 12 or 16 sheets of roll film thickness film, each with a long paper tongue. You changed sheets by pulling the tongue out which moved the film exactly the way pulling the white tab does on the classic Polaroid pack. The difference was that you didn't get the second yellow and black tab to pull the film and print sheet through the processing rollers, which didn't exist. The exposed sheet just stayed at the back and the next shot was taken.
When all were exposed, you opened the pack in the dark, removed the stub of the paper tongue, and processed the sheets. Some people would "rob the pack" to remove a few exposed sheets before the whole pack had been exposed, but this was poor practice, problems with film flatness on the remaining shots would be common.
The standard film size/packaging numbers were 520, 522, and 523 for the 2-1/4 x 3-1.4, 3-1/4 x 4-1/4, and 4x5, sizes respectively (
http://en.wikipedia.org/wiki/Film_formats). A pack of Tri-x in 4x5 would thus be marked TX523, just as 35mm Tri-X in the standard cassette becomes TX135, where 135 designates that the film is in the standard cassette.