Also with SFF warheads the penetrator is not ideal because it is still relatively soft and relies on speed and mass essentially for penetration as it does not have a point to concentrate its penetration energy on.
Also... the 30mm cannon on the Apache helicopter has a 30mm diameter HEAT dual purpose round taht can penetrate 50mm of armour so rather small calibre rounds can still use effective HEAT warheads.
The problem of course is that 50mm penetration is not generally enough for most modern armoured targets from any direction... a top attack munition generally goes for 100mm+ penetration to be useful.. but a 57mm warhead from the RPG-16 could penetrate 200mm plus of armour so it is not impossible... and with a suicide drone it wont be spin stabilised like a 30mm cannon round so its penetration should be even better.
The Bofors Bill-2 actually has an oval shaped downwards firing HEAT warhead to penetrate the roof structure of armoured vehicles... they don't have to be round and facing forward...
In fact the road side TM-83 is a circular mine with a diameter of 250mm... you could mount it on a UAV horizontally at the wing root facing downwards... the whole mine is about 20kgs but it blows a hole 80mm wide in teh side of a tank up to 50m away that will penetrate 400mm of armour which means it will penetrate the roof of any target... Just a simple system to fly the mine over the target could be used with perhaps a laser for precise aiming.... for a Russian weapon you could aim for the turret bustle of western tanks... a guaranteed kill, or perhaps the engine deck for a 100% mobility kill...
On a bigger UAV that flys higher you could have a stack of them and release them in flight with tail fins to steer them towards targets and at 40m altitude boom... what could trophy do to stop that?
You could make them smaller and lighter for a 200mm penetration if you want... perhaps using a cheaper HEAT liner material so they can be made in enormous numbers and add a mechanism so that if they don't detect a target on the way down when they land they flip over pointing their HEAT warhead directly upwards and the next heavy vehicle rolls over it and boom in the guts... belly armour is generally weaker than roof armour... and an 8cm diameter plasma beam coming up through the floor beaming anything in its way to heaven... something to fear from a crew point of view.