the file weighs a lot

File size and image size are two different thing.  File size depends on File format and what  the file format Support and how well the file's  image data data compresses,  File size will  vary with image detail data.  More details request more information  layer content also varies with layer type.  File size differences can be huge.

An image with a 1080px by 1080px canvas is a  1080*1080=1,166,400px image in 8 bit  RGB color each pixels  requires three 8 bit values one fr red, one for green, and one for blue so 3 bytes per pixel.  Your image size is 3,499,200 bytes 3MB.  Your file size can  vary all over the place.  It depends on content and file type save options.

Here I save a 7 layer 1080x1080 document as a psd  that has no image detail  the documents layers it save as a 111KB PSD file.  I then added details in all 7 layers that saved as a 25MB PSD file.  I then converted the 7 layer to Smart Object layers that saved as as 70MB PSD file.  You can see in the Image Size Dialog the the document image size is a 3MB 1080px by 1080px image.

I showed a rage of file sizes  111KB to 70MB I'm sure I could add more data to the document.

Why do you find your 16MB file size excessive? A flat file version should save  under the 3MB Image size however Meta data cans bulk up the file size with ancestor data.  The new Jpeg I saved because of all the image detail there is in it the size is near the 3MB canvas image size.  8 bit color supports 16,777,216 colors if the image was a spectrum of all possible colors  you would need a much larger canvas and no device can display the full 16,777,216 colors 16 Bit Color supports  281,474,976,710,656 I can not comprehend that number but I would like to see it in my bank account but then  I could not spend that much money I would have to give it away to out do Bill G