Thanks for researching further. It's interesting that it seems to be DE related. However it must also be related to some change in VirtualBox 4.3.28.
I found the 'xdpyinfo' command when I googled how to check my DPI. I have no idea how or where it gets/calculates it's information, or why it would do it differently in different desktop environments.
Yes I agree that if the DPI was really changed things like images would be distorted, but VirtualBox can't actually change the DPI of my physical screen where those images are ultimately displayed only what it reports is the DPI, so the only things that get affected are those things which are calculated based on the DPI, such as fonts where the pixel grid for a particular point size gets calculated, and probably the font information for a particular font size.
I only really noticed the difference in one application which I'm guessing was basing the width of some items based on the information from the font.
Actually this was interesting, while xdpyinfo (w/ VB 4.3.28) reports the screen resolution differently when I resize, it reports the screen physical dimensions as unchanged!
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xdpyinfo | grep 'dots\|pixels'
dimensions: 1902x939 pixels (270x203 millimeters)
resolution: 179x117 dots per inch
xdpyinfo | grep 'dots\|pixels'
dimensions: 859x872 pixels (270x203 millimeters)
resolution: 81x109 dots per inch
And it seems that 4.3.26 also has different behavior (and not exactly what I expected) when I resize the window.
What I would expect is that the VirtualBox display DPI is constant based on what the Host display's DPI is. Resizing should not change that. I would expect the resolution (# of horizontal and vertical pixels) to change when the window is resized to whatever the size of the window is. I would expect that VirtualBox would use the DPI and resolution to calculate the reported physical dimensions (which xdpyinfo seems to report in millimeters) that it communicates to the guest VM.
Here I've run xdpyinfo after resizing the VirtualBox 4.3.26 VM Window several times (from a different computer, but still Win7 host, with a guest OS of LM 17.1 KDE 32bit).
Note that the DPI stays basically square, but changes as I resize (which I would not have expected). Also see that when I shutdown and restarted the guest OS, the DPI became 96x96 again.
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xdpyinfo | grep 'dots\|pixels'
dimensions: 2369x1310 pixels (517x286 millimeters)
resolution: 116x116 dots per inch
xdpyinfo | grep 'dots\|pixels'
dimensions: 1687x1310 pixels (289x225 millimeters)
resolution: 148x148 dots per inch
xdpyinfo | grep 'dots\|pixels'
dimensions: 1687x1036 pixels (288x176 millimeters)
resolution: 149x150 dots per inch
xdpyinfo | grep 'dots\|pixels'
dimensions: 1687x843 pixels (262x131 millimeters)
resolution: 164x163 dots per inch
xdpyinfo | grep 'dots\|pixels'
dimensions: 1687x1310 pixels (261x202 millimeters)
resolution: 164x165 dots per inch
After shutdown and start up
xdpyinfo | grep 'dots\|pixels'
dimensions: 1687x1310 pixels (445x346 millimeters)
resolution: 96x96 dots per inch