It is true that the larger a box is, the longer it will take to heat up or cool off. But everything we do has a risk vs. benefit ratio that we have to balance as best we can. Otherwise, I would use a giant tropical fish box for one hatchling, even though it would cost almost $100 to ship. But it would be safer than a medium size box. And I think that a small box CAN be as ALMOST as safe as a medium box, depending on how / when it is used and how it is packed.
I look at the high and low temps in Memphis and the destination city, taking into account my own temps (I drive down to the FedEx office just before they close, so my temps are not as important as the others). And I take into account where it will be delivered (I try to encourage my customers to pick up at the FedEx office in the morning for the safest option) and the time it is likely to be delivered or picked up. After taking all of this info into account, I use my best judgment based on my experience as to whether it needs a heat or cold pack and what size box it needs. With corns, I can err on the side of being a little cool, rather than too warm. So I often don't need a heat pack when it will be picked up at FedEx and the temps are not TOO extreme. When shipping boas or pythons, it is riskier, since they can't be exposed to chilly temps as corns can. So I more often have to require my boa / python customers to pay for a larger box, compared to those buying one or two hatchling corns.
It is true that FedEx COULD totally mess up and send them to the wrong state, or who knows where? When that has happened (rarely), my packing has usually withstood the test. There have been a couple of times when NOTHING could have withstood their "test".
In short, I feel that an experienced shipper who is very careful can quite safely use small boxes in many (not all) cases when shipping just one or two baby corns. But it takes experience, time, and care to know which times are the wrong times to go for the small boxes. Shipping is like a whole separate business, and it is not easy to figure out the cheapest way that is still safe. For those just beginning to ship, I would suggest the extra safety of a bigger box even when not totally necessary. But I wouldn't make a blanket statement against small boxes in all cases.