I thought adding

-debug "All"  -log "%d      %f      %e "   
or
-debug "Coder,Policy"  -log "%d      %f      %e "
[download]

Would help you duplicate the calls convert makes, but it is not so helpful on my version (ImageMagick 6.5.6-5 2009-09-25 Q16 OpenMP)

Policy      IsRightsAuthorized      Domain: Path; rights=Read; pattern
+="somethumb.jpg" ... 
Policy      IsRightsAuthorized      Domain: Coder; rights=Read; patter
+n="JPEG" ... 
Policy      IsRightsAuthorized      Domain: Path; rights=Read; pattern
+="somethumb.jpg" ... 
Coder      ReadJPEGImage      Interlace: nonprogressive 
Coder      ReadJPEGImage      Data precision: 8 
Coder      ReadJPEGImage      Geometry: 105x78 
Coder      JPEGSetImageQuality      Quality: 75 
Coder      JPEGSetImageSamplingFactor      Colorspace: RGB 
Coder      JPEGSetImageSamplingFactor      Sampling Factors: 2x2,1x1,1
+x1 
Policy      IsRightsAuthorized      Domain: Coder; rights=Write; patte
+rn="JPG" ... 
Policy      IsRightsAuthorized      Domain: Path; rights=Write; patter
+n="cnv002.jpg" ... 
Coder      WriteJPEGImage      Image resolution: 72,72 
Coder      WriteJPEGImage      Interlace: non-progressive 
Coder      WriteJPEGImage      Quality: 75 
Coder      WriteJPEGImage        Input sampling-factors=2x2,1x1,1x1 
Coder      WriteJPEGImage      Storage class: DirectClass 
Coder      WriteJPEGImage      Depth: 8 
Coder      WriteJPEGImage      Number of colors: unspecified 
Coder      WriteJPEGImage      JPEG data precision: 8 
Coder      WriteJPEGImage      Image colorspace is RGB 
Coder      WriteJPEGImage      Sampling factors: 2x2,1x1,1x1
[download]