These questions made me curious so I just did an experiment. Vital statistics are as follows: Beautiful clear day, beginning 12 noon and ending 12:30 CDT in the far western corner of Kentucky, zone 7a. Garden established for 30+ years with Crimson Pirate planted in about 1982 and ending with a lovely spidery lavender a year ago that came from Bluegrassmom, who lives also in KY but in a cooler zone. Others planted mostly in the late '90s, but none newer than the one from Teresa last year. At this time of year, there are only a few blooms left in my garden, it's winding down. Crimson Pirate always blooms first, beginning in early to mid May. All daylilies are in ground and have been for years.
The first photo includes a ruler at the bottom, but then I realized I had forgotten Crimson Pirate and in the meantime had somehow chopped off his head, but still a good comparison. I added him to the second picture but forgot to replace the ruler. Imagine that, a writer/artist/retired humanities teacher trying to be a scientist?? Using the other side of my brain is a bit difficult, it rarely ever gets used. SO, here we go and a brief synopsis will follow, not very scientific either, but best I can do.
In the second photo from left to right, foliage cut from same (but approximate) distance from tip:
BIg yellow NOID
Copper seedling (probably Crimson Pirate x yellow and could be MaryTL
Hot Pink Noid seedling , could be also from CP
BIg Huge RED and beautiful NOID
Small spidery red
Big wine/purple/near black NOID
Bluegrassmom spidery lavender
Small compact red
I have many NOID seedlings -- as I explained in another thread - because my retired pediatrician uncle dabbled in hybridizing for a few years in the late 90s. I know he used Crimson Pirate as a parent often. Most of them are tagged in my own system as Adams/copper #1 or Adams/hot pink #6 or whatever, with a parent named if known. Also, last year's ragged winter destroyed tags and I'm not disturbing their roots digging to find buried tags.
And my very scientific synopsis . . .
Daylilies that are well established in ground with similar or same existing conditions -- water, soil, sun -- are not going to have very much variance in foliage width size, only fractional differences at best. Perhaps the smallest minis or some spiders might show a difference, I have none of them, though no difference in my stellas or in the spidery lavender that BGM sent.
However, Seed, I would note that there is a very high chance that those CPirate plants are tissue cultured and I've read that tissue cultured plants have been found often to be inferior (read grassy) to the natural divisions that come from the hybridizer. Mine is very old and came with me from the mountains of SE KY, as did those of my uncle.
I mention 'well established' because of this particular situation: See the strand of foliage that is labeled PPP above and to the far right? That is a plant that is said to have Purple Paw Print in its lineage. I don't know, but it was sent to me in 2009 from a friend in California. When I received it in early spring, it had the most gorgeous big wide dark green foliage I had ever seen. I was so excited. It bloomed and grew lush that first season in KY. It's second season I noticed its foliage was a bit smaller in width, it seemed to blend in well with those others near by that had been here for years. Now, this fifth year of its life in KY, there is no difference in it and the others in my gardens. Even the bloom has changed somewhat:
A couple weeks ago on the left (after a rain) and in 2009 on the right
So for me, there is little to no obvious difference. I tend to believe it's because I live on old farmland with rich soil and I have a natural underground spring that flows from a little hill above my back yard where the daylilies all grow. Same sun, same soil, same water amounts and several years of in ground growth = very similar foliage on most daylilies. That could also explain why you won't find much on the subject of 'narrow or broad leafed daylilies' online. I couldn't find anything either.
Or so says the little used scientific side of my brain.
Now I need a nap.
Edit: thought I might need to clarify and emphasize that this info pertains to my little corner of zone 7a.