Loudness is a complex issue. Most meters can read dBA or A-weighted decibels. The A-weighting noticeably rolls off the lows and some mids to make the reading better correspond with subjective loudness. I often use the example of a railroad locomotive at 80 dBA (50 feet) and a high-hat cymbal at 80 dBA (measured at 2 feet). One has a huge amount of low end and WAY more acoustic power, the other only high frequencies and less power. But two juries of listeners would describe each having similar overall loudness. It's not perfect, but is the best that the industry can do.
A sound meter set to read dBA will give similar readings for any size acoustic guitar, as we saw. The dBA levels were quite similar even though you could tell that each larger guitar had a fuller sound with more bass. Using dBC or the the C-weighting (without the A weighting corrections, or a more linear number) would show noticeably different levels for guitars with more bass.
There is also the variability of each strum. If you watch carefully, the level bounced around quite a bit. I told you this was complicated! This Wikipedia page explains it in more detail, with pictures:
https://en.wikipedia.org/wiki/Weighting_filter