Size of coffee grind question

Husband grinds the beans to powder in a burr mill for a Chemex drip. Bought some beans from a local roaster who said NEVER grind finer than mid range to avoid over extraction. I was too rushed to ask over extraction of what.
Is there any science here other than more surface exposure to the water and temperature effects from over grinding?
What differences have you noticed with grind size changes and why ?

The brewing process should determine the grind. There are a few types of brewing where grinding to a fine powder is recommended, such as espresso. Last year I went to a local Greek Festival, where they served "Greek Coffee", something I was unfamiliar with. Very finely ground coffee was stirred into a pot of boiling water. The pot was removed from the heat, but stirring continued until the coffee was poured into cups. This left a layer of fine grounds in the bottom of the cup. If a very fine grind is used for drip makers that use paper filters, they could coat or plug the filter, causing the coffee to overflow the filter and getting grounds into the carafe. A coarse grind is recommended for a French Press. This allows the plunger screen to push the grounds to the bottom without passing through the screen. Extraction, I believe is more a function of temperature and time. Oils and some bitter compounds are extracted when using a steam espresso maker. The actual brewing temperatures of drip and French Press are several degrees below the boiling point of water.

Is there any science here other than more surface exposure to the water and temperature effects from over grinding?

As I understand it, that's the science. The effect on your cup is direct cause and effect: if the grind is fine then the water extracts more of everything the bean has to offer. You get fuller flavor and richer mouthfeel (more oils) but you also get more bitterness, even a metallic twang. Hence there is lots of fiddling around with how much water and how long it remains in contact with the grounds. With a coarser grind, you worry less but you also can't rush it -- my cold-brew method takes 16 hours.

Our method is the ole drip method. We pour boiling water over grinds in a funnel into a carafe. For this method we grind between 27 and 33 seconds in a whirly bird grinder. The difference is the amount of oil on the beans. The drier the beans the shorter the grind.

The variables are grind size, water temp, bean roast, amount of water to beans, and exposure.

Espresso is typically fine grind, hot hot water (195 to just off boil), and short exposure time (30sec)

Pour over might be a mediumish grind, hot water (some say 30 seconds after taking off boil...205F), and longer exposure (4min)

Darker roasted beans usually want less exposure.

So the good flavors. good solubles from the beans typically are expressed faster than the bad flavors / longer chain molecules (bitterness, astringency)

Brew too quickly, have too large of a grind, or too cool waterand you end up with thin, sourish coffee.
Conversely if it's too slow, too find a grind, or too hot and you get an acidic harsh (over extracted) brew.

With a ChemEx, many will pour a short pour of water over the beans, and let the beans bloom (release CO2) before pouring the rest of the water in a steady stream. Letting the CO2 out first allows the water in to the bean and extract more cleanly.