When they rob the bank in Hell or High Water they say:
... none of them in bundles, just the loose cash.
Why do they only want loose cash and not bundles? Why do they only steal small amounts of money at a time?
Firstly, bundles of cash can contain dye packs....this is specifically mentioned in the movie (they are referred to as "ink packs".)
A dye pack is a radio-controlled incendiary device used by some banks to foil a bank robbery by causing stolen cash to be permanently marked with dye shortly after a robbery.
In most cases, a dye pack is placed in a hollowed-out space within a stack of banknotes, usually $10 or $20 bills. This stack of bills looks and feels similar to a real one, with technology allowing for the manufacturing of flexible dye packs which are difficult to detect by handling the stack.
Deployed dye-pack - (By Colin Brown from Silver Lake, CA, U.S.A - IMG_5096.JPG, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=4322776)
Secondly, bill packs might contain bait money where the bill numbers are recorded making them significantly more traceable.
As for the amounts taken...they are limited to time and they can only grab what is in the teller's drawers and relatively low value bills at that (for the reasons stated). $100 bills are tracked I believe...at least that's indicated in the movie.
Also, stealing too much will draw the attention of the FBI rather than local law enforcement.
The head ranger believes they have a plan...
"They're trying to raise a certain amount, that's my guess. It's gonna take a few banks to get there."