It is my understanding that
Max Link Length x Days and
Largest Field MUs x Days treats the
x as multiplication and shows the largest total value of that equation that you've acquired rather than only showing you the value from your largest link or field. So, if you had a small field for several days as opposed to a large field for a single day, the smaller field may be the value represented there.
Using my own stats as an example:
- Longest Link Ever Created: 8 km
- Largest Control Field: 368 MUs
- Max Time Link Maintained: 12 days
- Max Link Length x Days: 55 km-days
- Max Time Field Held: 1 days
- Largest Field MUs x Days: 32 MU-days
Here, my maximum time maintaining a link is 12 days and my longest link ever made is 8 km. I know for a fact that the 8 km link didn't last more than 3 days, so the 55 km-days value does not correlate. My maximum time maintaining a field is 1 day and my largest field is 368 MUs. That field only lasted about 2 hours which is close to the value of 32 MU-days (more like 30), but I'm also sure that the value was 32 MU-days prior to creating my largest field. (I also suspect Niantic doesn't calculate partial days, but I have no evidence to back that assumption in this particular case.)
As for the aside in your question asking why they don't give you a statistic specifically about your performance with your longest link or largest field, I imagine it'd be out of the scope of the statistics. Every statistic is always either increasing or remaining static. A statistic that showed your performance with your longest link or largest field could potentially decrease as you create longer links and larger fields with more risk of being broken quicker. The statistics seem to cater to the "these numbers are my numbers and I will make them big" appeal.