Hi, welcome to meta! 
I think your issue is that the lightbox is being applied to your images, making them use the full width. I’m pretty sure it would not apply to images that small before, so that may be your reason. Also, I looked at the CSS, and there were no recent changes on the [details] tag. 
Long story short: here is some CSS that you can add to your theme or a new theme component:
.cooked details[open] .lightbox-wrapper {
display: inline-block;
}
Then, you should get what you want:
As a side note, I think the image should be inline in [details]. That’s the default behavior outside that tag, and it would give the user the choice inline/block.
EDIT: As Moin found out, that the lightbox is applied to small images is related to: