The graph of the profile's gamut volume shows you three things: (i) the size of the gamut volume, (ii) whether there are kinks that could be indicators of discontinuous performance of colour rendition, and (iii) the shape of the gamut (the areas of the colour spectrum capable of more or less saturated colour rendition).
The four most important quality characteristics that you don't get from looking at that graph, or in fact from an unaided visual comparison of your printed colours relative to X-Rite's colour checker chart are: (i) whether the profile is allowing for accurate rendition of the file values on paper, (ii) the smoothness of the tonal gradations on paper where the file numbers indicate they should be smooth (skies, skin tones and the like), (iii) whether the shape of the gamut is well-suited to the colour characteristics of your photographs and (iv) the quality of shadow and highlight detail achievable with your profile, especially for Black and White prints where this characteristic would be most noticeable. While these characteristics depend heavily on the kind of paper and ink you are using, the profile is a key participant in the whole set of conditions that determine print quality. To assess these other factors you need other methods of analysis than those provided in X-Rite software. You can check several of my more recent printer/paper reviews on this website to get some ideas. But cutting to the bottom line of your observations to date, let me confirm what Doug Gray said above: the size of the gamut volume doesn't tell you much about the overall quality of a profile. I have produced custom profiles whose gamut volumes are a bit smaller than those of the canned profiles, but whose colour rendition is more accurate in my printing conditions. I'll take the accuracy over the variances of gamut volume any day.
(Typo corrected)
