BG I & II set the standard usage for barrels, which is a hiding place for anywhere from 5 to 30 gp. That is it.
For setting things on fire or whatever, D&D provided rules for oil flasks. Based on this discussion, it seems to me the issue is really that Larian simply needs reduce the size of their oil containers. According to the old DMG, a single "grenade like missile" flask of oil should be about 1 pint, or 16 ounces. Now, if there were a magic spell to make styrofoam,things could be very different ...
