GetOutlineTextMetricsW called with a default OUTLINETEXTMETRICW struct+size fails with. ERROR_INSUFFICIENT_BUFFER.
GetOutlineTextMetricsW after a while ends up in NtGdiGetOutlineTextMetricsInternalW (gdi\ntgdi\font.c), which is calling IntGetOutlineTextMetrics (gdi\ntgdi\freetype.c) to query size needed.
This size returned includes the font name + family name sizes, in addition to the size of the OUTLINETEXTMETRICW struct.
NtGdiGetOutlineTextMetricsInternalW responds by deciding "The buffer supplied is too small", and returns a failure.