I see the same behavior on macOS, albeit the workflow is a bit different. I changed the standard OS screenshot format to jpeg (via the terminal), to save storage space.
I take a partial screenshot to the clipboard. In Finder → Edit → Show Clipboard I can see that the file in the clipboard is a jpeg-file. When pasting that file into Logseq, it seems to actively convert it to a png file, increasing the filesize manyfold. Taking the screenshot to a file and dragging it into Logseq keeps the correct filetype. Other apps like Word and Pages don’t behave this way and import the jpeg-file from the clipboard correctly (the document file-sizes increase only modestly compared to the same screenshots as png-files).
I don’t want my hard drive/ icloud drive filled up unnecessarily, so I guess this is a bug. My version is 0.8.0 and the OS is up to date.