The conversion that people expect to do more than it does
A reasonable-sounding assumption is that converting a JPG to PNG — a "better," lossless format — should improve the image somehow, maybe cleaning up compression artifacts or sharpening blurry detail. It doesn't, and understanding why matters: once JPEG's lossy compression has discarded some information to shrink the file, that information is gone. Converting to PNG afterward simply preserves whatever's left, exactly as it currently looks, blockiness and all — you end up with a losslessly-compressed copy of an already-lossy image, not a restored one.
This isn't a limitation of any particular tool — it's a fundamental fact about how lossy compression works. No format conversion, however good, can recover detail that was already thrown away in an earlier step.
Where converting to PNG genuinely helps
The real value shows up in what happens next, not in the image itself. If you're feeding the image into an editing pipeline with several more save-and-edit rounds ahead, converting to PNG first stops the cycle of repeated JPEG re-compression, where each additional save discards a little more detail. Working in PNG for the remaining edits, then exporting to your final format at the end, means you only take one lossy hit instead of several compounding ones.
PNG is also often simply what a tool or platform requires — a design app that expects PNG input, a template system with a strict format requirement, or a need to add transparency to part of the image afterward (which requires PNG or a similar format, since JPG can't represent transparency at all).
The size increase is normal, not a bug
Converting a photo from JPG to PNG typically makes the file noticeably larger, sometimes dramatically so — and that's expected, not a sign something went wrong. PNG's lossless compression preserves every pixel exactly, while JPEG's lossy compression achieves its smaller size specifically by discarding information PNG has no equivalent way to throw away. For a photograph, that trade generally works against PNG on file size while working in its favor on preserving whatever detail actually remains in the source.
When it's not worth converting at all
If your JPG is already the final image — nothing further planned, no transparency needed, no additional editing rounds ahead — converting to PNG mostly just makes the file bigger for no practical benefit. Save the conversion for when there's a genuine downstream reason to need it, rather than converting by habit whenever a "better" format is available.