I found some of our unresolved 2007 conversations that had gone "offlist" and here was one of the theories I came up with at the time:
"Too bad the portable subs are blocking our view of the building, but from the brickwork it does look much smaller than the building today. OTOH, that tiny picture in the book is undated, and from the little bit of vacant countryside that's visible in the background it could be very old, at least prior to 1928. By that year's Sanborn map, the wide open area "behind" (east of) the substation in the picture had houses and outbuildings on every lot...
"That's why I wonder if needing the portable subs might be an indication it needed more capacity at the time the pic was taken, and was thus was expanded later on."
Here's three images cobbled in one; the earlier "wide open area" I referred to above is right where I've superimposed the aerial photo: