It's different planes and different flying conditions. Picture a plane trying to do a quarter loop. Without sufficient energy it will not make it to 90 degrees (pointed straight up).
Now, considering most GA planes have a thrust to weight ratio of around 0.20 to 0.25, what affects how far into the "loop" we go before stalling? Do we "mush" or pitch down sharply?
Energy is entry speed, so if we dive and then pull up at full power (zoom) we will go higher into the loop, maybe even completing it. But if you don't make it to 90 degrees, the plane loses airspeed, stalls (with continued elevator deflection), sinks, and pitches sharply down.
Two factors contribute to pitch down. At full stall center of lift moves back to 50% chord and tail volume (as plane sinks) will "flip" the nose down.
Now picture entering the loop with no power or zoom, your plane does not nearly go as high to vertical, it simply loses airspeed and does a much gentler "mush".
Other factors include elevator authority. The B-52 bomber has a comparatively "weak" elevator compared with an Extra 300. You don't want to stall one of those.
And light weight, which enables a much sharper loop entry.
Also placement of the CG. Many trainer aircraft, including the Cessna 172, as set up for students with a forward CG (within limits) to "weaken" elevator pitch up authority and encourage "mush".
But even with the 172, if CG is aft, stall characteristics can change dramaticly.