Rows with an integer id and a few integer or bit flag columns or maybe a short short fixed width char column or two and a few well though out indexes, is a way different beast compared to something with dozens of big varchar() or blob columns (or worse still, json or xml text columns with un-indexable key/values inside that you need to parse out before you can run selects on them...)
Ah, I think I understand now. It's not the amount of data (rows), it's how that data is structured (columns). I thought you were saying that number of columns was more important than number of rows, which clashed with my naive intuition.
Modern RDBMS's can support JSON and XML as native data types, i.e. you absolutely can index within them and they have facilities to speed up the parsing.
Yeah. But not if you've just shoved them into text columns.