So it looks like if no guifont is specified in your vimrc, gVim will use some built in default font and it would not be reflected in guifont -- set guifont is empty.
But it turns out it is possible to change the size of your default font -- use guifont without name, just size:
whatever your default bitmap font supports
I found a workaround. Manually copying the entire vim-airline tree under the $HOME/vimfiles/pack/plugins/start/ did the trick, as the vim-airline plugin was then able to be loaded at startup. For some reason, simply installing vim-airline via Vundle doesn't make it able to be loaded on startup on both of my setups, unlike other packages.
Update: I also ...