ImageMagick - Smaller resized image is a much larger file size

In ImageMagick, I'm running a:
convert -trim -density 200 myfile.tif -resize 70% myconvertedfile.png

If I remove -resize 70%, the image size is only 78,527.
However, when the file is stored at 70%, the file size is 860,504

What makes the smaller image a larger file size, and is there a way to decrease the file size without quality loss?