Departing from a fairly busy Euston this morning.
Notable that most people including those standing between the smaller boards were looking at the big one in preference.
As
@All Line Rover says above, the big board is more legible than the small ones. You have to stand quite close to the small ones in order for the text to be the same apparent size. And at that distance the text on the small boards is visibly more pixelated. It is sharper on the big board. These images below are both enlarged from the photo above. I'd say the effect is more pronounced in real life.

This says something about priorities doesn't it, that the passenger info boards were specified at a lower display quality than the one whose purpose was to blast us with advertising?
My train (Avanti to Machester) was called 10 minutes before departure. From 20 minutes pre departure I could see it sitting there on the platform, doors open, no visible activity happening. It seems that the platform announcement was triggered entirely by the 10 minutes timing, rather than by anyone saying, train's ready now, you can let people on.