Is this achievable by the size of the craft itself? Or the fact that these bigger planes have 1000s of engineers behind them optimizing every possible factor to squeak out the best possible lift to drag ratio?

It's the overal effort that goes into the design, combined with a different design goal. Airliners are specifically optimised for low drag because it saves fuel. A Cessna 172 is a design from the 1960s that wasn't much optimised after that, and the design goal was stability and benign flying not the lowest possible drag. Later similarly sized small aircraft like the DA40 and SR20 are better at low drag because they're computer designed and fuel cost became more of a thing in modern times.

For airliners the retractable gear saves tons of drag, but they also have things like winglets, and lots of smaller things like optimisation on the wing-root (airliners have smooth transitions here, trainers are sort of just square) that all reduce drag.

An airliner is also much less friendly to handle in a stall condition, while a trainer almost recovers on it's own. And similar for yaw stability, if you get the rudder a bit wrong in a trainer nothing much happens, while an airliner will typically start to oscillate. Both of these are solved on the airliner by electronics, a stick shaker or pusher to avoid the stall and a yaw damper to enhance stability. The Cessna 172 doesn't have any electronics involved in flying, only for navigation and those are optional.