Image size, file size and DPI: three different things
An image has pixel dimensions, an encoded file size and sometimes print-density metadata. Changing one does not necessarily change the others.
Pixels describe detail
A 1200 × 800 image contains 960,000 pixels. Resizing to 600 × 400 reduces the number of pixels to one quarter. Increasing the dimensions cannot restore detail that was never captured.
Kilobytes describe storage
JPEG and WebP quality settings change how efficiently the pixels are stored. PNG is lossless, so a quality slider does not work the same way. For a strict upload limit, try target-KB compression and inspect small text afterwards.
DPI describes a print instruction
At 300 dots per inch, 1200 pixels correspond to four inches. Updating DPI metadata does not add pixels. Print applications may ignore that metadata, so check the print dialog too.