Instead of worrying whether you have enough pixels, just try it out. The proof is in the final printed image. Don't let the technical discussion about resolution deter you from printing your photos.
I have had decent 4R from my older 1.3 Mpixel Fuji camera. In my newer Ixus 400, I blow up my pictures to 8R routinely and the resolution is at 180 PPI. Looks good in prints. Will it be better at 300 PPI? Probably it will be technically clearer. But the essence of a photo wouldn't change one bit, unless all you are printing are pics of rulers and resolution charts
As for your query about why your 1.3 Mb Jpeg file (I assume), it could be that your camera saved the image at a higher compression, but you saved your in a higher quality setting instead. Not knowing your camera mode or post processing workflow, that's my guess. I had experienced cropping a 1.3Mb Jpeg in PS and the resulting saved file was 2.2Mb.