If you look at Figure 3 (and section 3.2.1), you'll find they're reporting the number of papers in percentage of the total number of papers. So, relative size.

If you look at Figure 3 (and section 3.2.1), you'll find they're reporting the number of papers in percentage of the total number of papers. So, relative size. The increase given in the abstract is that between 1990 and 2014.