How did you 'insert' the images into the presentation in the first place?
Going by the description, it appears your images have been inserted as OLE
objects which accounts for the increased file size and a 73 MB oledata.mso
confirms it. oledata.mso stores OLE object information for the objects
present in the presentation when you save the file as HTML. The 5 MB PNGs
are the images that used to represent the OLE objects in the presentation.
You will need to use the Insert | Picture | From file... command to avoid
this file bloat in the future.
Image Importer Wizard: http://www.yqcomputer.com/