How this conversion works
Your JPG is drawn onto an HTML canvas in your browser at its original resolution, then re-exported as a PNG. Nothing is uploaded — the whole process happens on your device. A real JPG file, checked by this tool's own validation before any conversion is attempted, is accepted here: yes.
Why the PNG file is often bigger, not smaller
JPG uses lossy compression; PNG uses lossless compression. Converting a photographic JPG to PNG typically produces a larger file, sometimes substantially so — this is normal, expected behavior for this specific conversion direction, not a bug. If your goal is a smaller file, this isn't the right direction; Image Compressor or PNG to JPG (if you're going the other way) are built for that.
What doesn't change
Your image's pixel dimensions are preserved exactly — this tool only changes the file format, never the size or content of the image itself. Since JPG has no transparency, the resulting PNG's alpha channel is uniformly opaque across the whole image; there's no transparency to "recover" from a source format that never had it.
Privacy
Your image never leaves your browser. Nothing is uploaded, logged, or stored anywhere.