According to this tweet by Jeremy Crawford:
Unarmed strikes have always been weapon attacks. Here's where the confusion comes in: your unarmed strike (fist, elbow, knee, butt, etc.) is not considered by the rules to be a weapon the way a longsword is a weapon. But the rules let you make unarmed weapon attacks anyway.
This ruling is clear for features like a barbarian's Rage, but less so for something like the Enlarge function of the enlarge/reduce spell:
The target's weapons also grow to match its new size. While these weapons are enlarged, the target's attacks with them deal 1d4 extra damage.
An unarmed strike qualifies as a melee weapon attack, but does it qualify as a "weapon" under these circumstances?
It's relatively obvious for effects that specify a weapon as an object, like the Forge cleric's Blessing of the Forge, but this seems like a grey area.