Finer crush = greater surface area.
A very coarse crush takes longer to gelatinize and for the water to penetrate and for conversion to take place.
A fine crush, allows for faster conversion.
If you have a high temperature with a coarse crush, the Beta Amylase will be denature before it has much chance to work.
With a fine crush, the Beta Amylase is able to do some conversion before it's denatured.
If you mash at a high temperature, because you want a less fermentable beer, the Beta Amylase gets denatured quickly. Lets say by 5 minutes a fair amount is denatured, and by 15 minutes much of it is denatured.
With a coarse crush, thats not enough time for the Beta Amylase to break many bonds, because the water hasn't penetrated the grain.
But with tiny grains, water can quickly penetrate and with a greater surface area, there are more linkages broken, resulting in a more fermentable wort.
Temperature is the key factor in Beta Amylase denaturing, but that denaturing is not instantaneous. It takes 5-10 minutes (for arguments sake). With a very fine BIAB crush you will get more broken linkages than with a traditional mash with coarse grains.
BIAB with a fine crush can give fairly fast conversion, faster than a traditional mash.
Arclight, I respectfully disagree and second Gavin's comments on evidence.
Finer crush DOES not impact fermentability but TIME does.
We control temperature and pH to adjust the activity of the alpha and beta. This is independent of available starch, or "raw materials." In other words I can optimize enzyme activity even when no starch it present. The enzymes will just sit idle waiting for the raw materials. They will convert as fast as they can based on how accessible the raw materials are.
The time we pick for a mash is based on crush size. Crush size is how accessible the raw materials are to the enzymes. If you have a coarse crush, or in this case "standard", it takes more time for the starches to be accessible. If you use Beersmith and select a light body mash profile it will recommend a 90 minute mash at a low temp. This is to ensure you get max fermentablity. You could mash at 154 degrees f for 2 hours and achieve the same results.
Do a mash with normal crush for 30 minutes at 146 degrees. I bet you will get a full body beer. Do a mash at 154 for 2 hours and I bet you get a light body beer.
A finer crush just means the raw materials are more accessible so the time needed to get the same results is reduced. Remember the enzymes work as fast as they can based on pH and temp independent of amount of starches needing conversion.
If you do a finer crush but don't adjust your mash times then there is some risk you will not get the get the desired results.
I have no problem using a fine crush (.025) and doing a 30 minute mash and achieving a light or full body beer.
Also, there are actual scientific studies on the impact of crush on conversion time. You can google them and you will see the data confirms what I and others are saying.
Either way let's all continue brewing beer and having a great time doing it!
Cheers.