My guess is there are a number of issues leading to the greater utilization of oversized tubing in bicycle frame manufacturing.
Part of it would be the evaluation of stresses in the frame.
As evident from the slingshot frames above, for the most part with general riding, there is downward force on the two wheels and a stretching or tension on the downtube... EXCEPT WHEN THERE ISN'T.
Browse around used bikes or frames enough, or the
Wacky Thread long enough and you'll see a typical crimp in the top tube and downtube, just behind the head tube from a frontal impact.
For a road bike, these front impacts may be a fairly rare condition. For a MTB, it may be part of normal every day riding. Nonetheless, it is not fun to trash one's frame.
The slingshot MTB may be odd riding as it bumps over large limbs or over curbs.
The other issue that comes up is FLEX.
I think a lot of the flex issues are in the bottom bracket. One of the more recent innovations is to move from a T-shaped connection to the bottom bracket shell to a full width connection to the bottom bracket shell. Thus giving maximum support to the BB.
Many of the ovalized oversized tubes have a reverse ovalization at the point where they are welded to the bottom bracket for a wider weld point, or perhaps as in the case of the black bike posted by the OP, the downtube is mitered and welded to the bottom bracket shell and the seatube.
Ovalizing can also help with welding a tube bigger than the head tube to the head tube.
Around 2000, there were several aluminum frames made with teardrop tubes. Perhaps only a short-term fad, but the idea of teardrop tubes will return periodically. Plus, ovalized or teardrop tubes may be more aerodynamic for a downtube being exposed to frontal wind. The top tube isn't exposed to the same frontal wind as much.
Different parts of the frame may be subjected different stresses. The front part of the downtube likely is mainly subjected to a stress parallel to the frame axis. On the other hand, the bottom bracket would be subjected to stresses perpendicular to the frame.
You might request this be moved to the
FRAMEBUILDER'S SUBFORUM for more technical insight.