Ok, the attachment contains further information.
Annotation: I used a sampl for showing the problem. The target file now displays all images too big.
@Karellen: the html file contains no point concerning the image sizes.
It looks like this:
<p class="p-p4">*</p>
<div class="p-p4"><div class="g-fr"><img alt="" src="cover.jpeg" class="calibre2"/>
</div>
I'm viewing the file in the Calibre viewer.