I'm just wondering if this is a joke thread or something.
Pro mix is dried and compacted. Add water and it expands. True or false.
Yes, sorta. Most of the expansion comes from un-compacting it. Some comes from the absorbing of water, but not all that much.
Dry peat actually repels water and MUST be pre wetted before use. Using it dry from the bag is a typical noob mistake and then they blame their ignorance on the pro mix, peat moss.
BTW, over the years I've noticed peat moss expands by ~1/2. A 4cf bale when 'fluffed out' expands to a bit under 6CF. No where near the double some claim.
I've been making my own mix from scratch for over 40 years... I do a bit over 2cf at a pop (wheelbarrow full), mix dry till I get to the perlite. I'll use tap hot water with a squirt of Ivory liquid in a watering can to both keep the perlite dust down and to get the peat moss absorbing water. For the 15 gal (2cf), of mix, it takes 5-7 gal of this soapy water to get it moist. Moist enough to initiate the microbes for the 'cook' and moist enough to where it will absorb water when plants go in it.
The simplest way I can put it is, think of a dry mop vs a damp mop AFA picking up water.
Wet