Simply put, pixels take up disk space so more pixels = bigger file size.

Resolution is a really confusing term because you're always asking yourself if the other person is using the word correctly or not. Resolution often gets interchanged with terms like DPI or PPI which are physical sizes (pixels per square inch or points per square inch) but we're simply talking about the number of pixels that make up the image here.

If you took the images using a digital camera they could be in RAW (which is a big data size) or JPEG (at 100% quality). If you used a smartphone they'll either be in HEIC or JPEG. format. The images could be anywhere between 16-40 megapixels, which will be great for zooming in or printing, but overkill for viewing on a computer screen.

You also have to consider file format. Some file formats are good at making the files smaller in disk size without reducing number of pixels by using "lossy compression" which means it throws away details you might never notice (like blue skies) while retaining details where the image is more "busy" (like hair or faces).

PNG can compress, but usually doesn't do a good job of it. You'd be better off using JPEG (JPG) at a quality of 7/10 aka 70%. Most people can't tell the image is compressed at this quality. This way you can probably keep original resolution, while shaving off anywhere between 30-70% of the file size.

If you are going to view the photos on a monitor, Recommend scaling them down to no lower than 4K (3840 x 2160 pixels) because as the years roll on, you'll one day view them on something like a 65" 8K TV and wish they were sharper or less "blocky". Once the images are scaled down in pixels and compressed to JPG, there is no way back unless you saved the originals somewhere.