We’ve also come across this same issue – switching from Fly Dynamic Image Resizer to Better Image Sizes saw the memory usage on our server spike massively.
On further investigation we did also notice an enormous number of calls to bis_get_attachment_image_attributes() in the hundreds of thousands. This wasn’t proportional to the amount of traffic on the site.
We waited a few days in case this was down to image generation occuring for all our images again, however the memory utilisation was consistently high until we eventually disabled the plugin and everything returned to normal.
