There have been military munitions (usually cluster bombs with submunitions) specifically designed for cratering, in order to rend (for instance) an airfield's runway useless for conventional aircraft. The submunitions for this purpose are larger than you need, however, for two reasons. First, they need to crater paved runway surfaces (concrete or asphalt) and second, they can't depend on penetrating below surface level to crater efficiently; instead, they need to use much higher pressure to shatter, lift, and move the pavement and substrate from above.
An efficient cratering charge would be buried -- drilled in and the hole stemmed, ideally, or at least dropped into a hole and covered with soil. For air delivered munitions style charges, they would need to be designed to penetrate the soil.
This requires a munition that looks rather like an arrow -- fin stabilized (for simplicity, since you're apparently going to use millions or billions of them), with a sharply pointed, hard nose cone and a high sectional density, in order to push soil out of its way to the necessary depth. Further, the explosive needs to be the right type -- shattering power (like from straight dynamite or C-4) is less effective here than lifting power (meaning you want an explosive with high gas volume, more than one with high detonation speed). This is very different from what's usually packed in military munitions.
Once your "powder" is packed in the arrow-like penetrators, however, it can do the most efficient job only on unpaved, packed soil; if the soil is soft it will overpenetrate and produce less cratering (because it can't lift the broken soil clear of the crater) and if it strikes pavement (or concrete buildings, or other hard material) it won't penetrate enough and will produce shallow surface damage without much if any crater.
Therefore, any mining engineer could tell Bob that this isn't a job for a single, cheap to ultra-mass-produce munition, but will require at least three types -- one with less penetration (blunt nose?) for soft soils, one with the classic arrow design for packed earth, and a third with much higher sectional density and harder point to penetrate stone, concrete, and so forth.
To directly answer the original question, however, back in WWII, a satchel charge was commonly used for cratering, among its many other applications, and still produces less of a crater than you want -- and a common satchel charge size was 2.5-4.5 kg of TNT (the "satchel" was because this came with wire, one or more blasting caps, and an electric dynamo style detonator). If delivered 3-4 meters below ground in packed earth, a charge from this size to about four times this size might produce the size crater you want fairly reliably.