The Adobe ePub reader determines the number of columns to use based on the width of the screen in em units, i.e. the smaller the font, the wider the screen is when measured in em, and so the more columns used.
Changing the body font sizes in the CSS stylesheet and then rescaling all the other font sizes in proportion might affect the calculations, so if the publisher has done this (many do unfortunately) then the final size of the text on the screen where the number of columns change could be different for each book.
However you can quite easily change the underlying threshold font sizes that are used to select the number of columns to use by editing the Adobe XPGT stylesheet (usually called page-template.xpgt if the book has one, or add your own if it doesn't.)
You might also be able to use the XPGT stylesheet to force a particular number of columns to be used, I haven't looked into that.
Attached is a sample ePub that increases the font size thresholds so that I can get two or three columns on my Glo as the font size is reduced. I don't know much about the format of the XPGT stylesheet, I'm just going by what I have seen some publishers do. Adjust the values
"55em",
"40em",
"25em" to set the width that the screen has to be before more columns are used.
page-template.xpgt