Speed is of the essence when delivering fish, the less time you spend at the pond means you can get more deliveries done during a day. It's not like you jump in the truck and hit the road. It takes 1-2, maybe more hours to load the truck with the fish, then it takes at least an hour after returning to sterilize/clean everything. Add in the drive time between deliveries and sometimes you can only make 2-3 deliveries in a 10 hour day and that is if you eat lunch while you are driving. If it takes 45 minutes to deliver the fish at a pond, and 1/2 hour to drive to another pond, that is 3 1/4 hours right there out of the day to make 1 delivery. Add the time it takes to measure out the salt/chemicals that go into the fish haul tanks, the time it takes to fill/replace O2 tanks on the haul truck, etc, etc. there is a lot of time that goes into a single day of delivering fish. My fish delivery days are typically 12+ hours long..........
750 bluegill would have weighed between 40# and 100#, depending on the size of the fish. 4" Bluegill weigh approximately 40#/1000 fish, 6" Bluegill weigh a wee bit over 100#/1,000 fish.
Hauling rates in tanks could be up to 1#/gallon of water, depending on the size of the fish and how long they are in the tank. Depending on how the delivery truck was set up, they could have put the LMB, HSB and CC in the same tank. The bluegills in another and the Fatheads/Shiners in a 3rd tank, although it's much more common to run the minnows in separate tanks and weigh them out on-site. But they could have weighed/counted the fish before putting them in the tanks so the guy delivering the fish didn't have to do that at the pond, but then he most likely would have moved away from the pond and dumped the water - no point in hauling that water around with no fish in it. I agree with Sunil, the HSB are very sensitive to being crowded during moving the fish, and they wipe the slime coat off of their neighbors if crowded. Even netting them out of the tank you don't want to have too many in the net because of that.
Approximately 8# of fish displace 1 gallon of water. Unless the buckets of water had screw-in lids, there is no way to put 40# of fish in a bucket, and if you did you'd better be about 30 seconds to a minute away from first putting fish in the bucket to dumping them in the pond. I typically fill the 5 gal bucket about half full of water and then put fish in it, and I use screw on lids so the fish don't slosh out. To minimize the length of time at each pond acclimating the fish, we typically check water temp in the pond/holding tanks here, then adjust the water temp in the haul tanks so it mimics the pond water temp so we don't have to spend the time acclimating the fish at the customers pond. You shouldn't have more than a 3 degree temp difference for transferring fish, and it takes 20-30 minutes to acclimate the fish to a 10°F temp change. For big orders, you can pump water from the pond to the holding tanks on the truck, but it's not practical for small orders, and I will not do that period due to bio security reasons unless the whole truck is getting delivered to one pond because then the whole truck needs to be sterilized before loading any more fish. I've done this enough I can judge how much ice/no ice to put in the tanks to ensure that the water is the correct temp at the pond to stock fish without having to temper them much if at all.
Speaking of bio security, that is a big deal today for most fish haulers and rightly so. I use buckets to dip water out of the tank and use nets to get the fish from the tanks. The buckets that touch the tank do not touch the buckets that are used to stock the fish in the pond. The nets used to get the fish from the tanks do not touch pond water. If any of them do, such as a net wen stocking large quantities of fish into a pay lake where I can net from the tanks and turn, walk a few steps and put the fish in the pond, I sterilize the net(s) and tanks when I get back. Some hatcheries go as far as having you walk thru a sterilizing solution to sterilize your shoes/boots before you can walk inside the building.
I cannot find bluegill anywhere from my Arkansas suppliers that are bigger than 3 1/2". My suppliers said they are selling fish so fast that they don't have time to grow them larger, and that next year they don't expect to have the advanced size fish either because they are selling ALL the fish that they can grow - they can't keep them for the 2 years that is needed to get to grow to the advanced sizes. The largest LMB that is available now is 4".
They could have purchased the fish earlier in the year and grew them bigger, or grew their own. The hatcheries that I know of hatch the LMB and grow them larger on feed in tanks and have to grade them weekly by the 1/4" until they are around 3" then they can up the sorting to 1/2". If not they lose too many due to cannibalism.