I think you can resolve this by:
1. first inserting this code in your child theme:
.hentry .archive-thumbnail {
width: XXXpx;
height: YYYpx;
}
You will have to put the new width of your content space (instead of XXX and YYY).
2. For changes to work, you have to change the sizes of your thumbnails. You can do this in Settings – Media (in dashboard). Again, here at thumbnail, you have to put your new XXX and YYY measures.
Probably this changes are good just for images inserted from this point forward.
3. For changes to older posts, you have to force your theme to change your old thumbnails. (For every image inserted in media, the theme has saved some variations of the image cropping it in thumbnails.)
I successfully solve this with a plugin: Force Regenerate Thumbnails. This plugin will help you converting your old thumbnails to new measures.
4. If this do not work, you have to delete some lines in the blog pages .php. (index.php or Blog Page Template(page-template-blog.php))
Hope I understood the problem and this will help.