Not sure where there is a setting that drives this, as I thought the post settings state a maximum image dimension and anything larger gets resized to it. Somehow it seems some are being resized in a topic while others aren't (same topic post, same poster). The only place I see thumbnails mentioned is in Gallery settings and these aren't in or from Gallery but rather direct uploads to the forum topic.
Example de-ided with xxxxxxxxx
<img class="ipsImage ipsImage_thumbnailed" data-fileid="157996" data-ratio="66.67" style="width:216px;height:auto;" width="1000" alt="xxxxxxxxx" data-src="xxxxxxxxx" src="xxxxxxxxx" data-loaded="true">