The radius of the Fresnel zone formula gives resolution as a function of the dominant frequency in the spectrum. And indeed, the higher the dominant frequency, the higher the resolution.
In addition to what Way of the Geophysicist explained, I'd add specifically on Yilmaz that waht he is talking about is the importance of bandwidth. The plot you attached shows that with progressively increasing bandwidth, you are able to resolve better and better the reflectors in the reflectivity model (top panel) and eventually, with a bandwidth upwards of 10-50 Hz, even the reflectors separated by only 12 ms (third from the left in each panel).
To understand this I find it helpful to also look at Figure 1.1.29 [note 1 ], which shows a series of band-limited responses of the same reflectivity model as in Figure 1.1.30. Figure 1.1.29 demonstrates that if you increase the dominant frequency (the center of the narrow band range) only, you do not get improved resolution; you need also to increase the bandwidth as in Figure 1.1.30.
Figure 1.1.23 will also help understanding: it shows a series of zero-phase wavelets in which, with progressively increasing bandwidth, the wavelets are more compressed in time (hence higher temporal resolution). Notice that the amplitude (size) of the side lobes is decreasing as well with increasing bandwidth.
1 There's a google book preview and the three figures I refer to are included. Scroll to the book TOC from here and click on link to Ch1.