After investigating further, the issue started with 2018-08, likely with support for hi dpi displays. In addition to setting Xlights to Windows 8 compatibility mode, there is a High DPI scaling override setting in the properties menu. Check the High DPI scaling override checkbox and set the value to System or System Enhanced and the expand/contract icons can be set to the larger size. I'll run with this now to take advantage of the newer functions. Unfortunately it looks like this means the Hi DPI support is effectively turned off.
An observation..., when flipping between Hi DPI settings: 'Application' and 'System', the expand/contract icons do not appear to shrink proportionately. When set to 'Application' they shrink smaller. It would be nice if they stayed proportionate in size or larger. Reading small font is not as difficult as trying to get the mouse exactly over the layer expand/contract icon or the 'x' in the upper right corner of a window on the XLights workspace.
Thanks.