Converting HEIC to PNG makes sense when you intend to edit the photo rather than just share it. PNG is lossless, so repeated edits and saves will not progressively degrade the image the way JPG does. The trade-off is size: a PNG of a 12-megapixel photo can easily reach 20 MB or more.
JPG for sharing, uploading and storing, since photographs compress well and the quality loss is invisible at high settings. PNG when the photo is heading into an editor and will be saved repeatedly, or when a tool specifically requires lossless input.
HEIC is a highly efficient lossy format and PNG is lossless with no perceptual compression at all. A five to ten times increase is normal for photographic content, because photos have no large flat areas for PNG to compress.
Nothing is lost beyond what HEIC already discarded when your phone saved the file. PNG preserves the decoded result exactly.
Try the original unedited photo from your camera roll. Edited exports, Live Photos and burst frames sometimes use HEIC variants the browser decoder cannot read.