Yes it is the unit that is important in determining digging in success, not the sheer amount of men.
In the early part of WW1 neither side encouraged their troops to dig in as the war was supposed to be about movement, the most that was attempted normally was a "scrape" in the ground giving very basic protection or in game terms an IP.
Trench's were also just shallow narrow in nature also only giving basic protection.
So digging in is difficult in F14, where IP's and TRENCH's exist you should really value them and if you do try to dig in expect to only achieve an IP unless you spend a lot of time in the same hex, trench's as we imagine them where built from 1915 on.
From the notes......
"The trenches that were dug in 1914 were extremely primitive. These early field works were simply nice straight and narrow ditches that were rarely continuous and barely capable of covering a standing man. There were no traverses, were severely overcrowded, and were extremely vulnerable to artillery fire. Many of these early trenches were the graves of many soldiers as nearby exploding artillery fire collapsed the walls and buried the men alive as they knelt for protection. For this reason, trenches in France '14 do not have the high defensive ratings as they in no way represent the more elaborate trenches constructed out of experience in the later years of the war.
Improved hexes represent the piling of debris, man made or natural, to create a more defensible position. It also represents a hastily dug scrape in the ground or the simple act of creating a concealed position. An alternate term for these positions might be "breast works".
Given the low entrenching value, it is the designer's intent to mainly allow improved positions to be built, with it taking a lengthy amount of time to construct a primitive trench. Engineers will be vital in order to speed up the process and make it more likely to successfully construct fighting positions.
This decision is primarily due to the lack of training, doctrine or entrenching equipment that resided in the line units of 1914. This would of course change drastically in the following years."