As for the size: https://www.adobe.com/uk/creativecloud/design/discover/a3-format.html
The larger the print, the higher the typical viewing distance → the lower the print resolution (in DPI) can be. See, for example: DPI For Printing - Design Resources
Your upscaled file may be larger, but it is not detailed at all. Here’s a view at pixel level – you got ‘more pixels’, but ‘worse pixels’.
Compare that with the sharpness and noise level (or lack of the latter) seen in your signature – that is sharp at pixel-level, because it was created in high resolution, to start with):
