A field-deployed device (or a gateway to field-deployed devices that do not, themselves, have any data management need or capability) has to manage a modest amount of data. But an aggregation point (the cloud being the most obvious example) has to manage many times more data – possibly orders of magnitude more.
At the same time, I have to say that they might not be all that different. Some IoT systems are going to be closed, meaning the nature of the things making up the system is known, and these won’t require much scaling. For example, a building automation system for a small- to mid-size building would have perhaps 100s of sensors and 10s of gateways, and may (or may not) push data up to a central aggregation point. If there are just 10s of gateways, we can create a UI that connects to the database on each gateway where each database is one shard of a single logical database, and execute analytics against that logical database without any need of a central aggregation point. We can extend this hypothetical case to a campus of buildings, or to a landlord with many buildings in a metropolitan area, and then a central aggregation point makes sense.