mogrify makes filesize of image about 3 times larger, when image is reduced

Question

I have some png images:

0.png   -  665.3 kB  -   1589 x 2175
1.png   -  989.9 kB  -   1589 x 2175
2.png   -  138.7 kB  -   1589 x 2175
3.png   -  351.9 kB  -   1589 x 2175

and I want to make the height of images 2000 (smaller) so I give this command:

mogrify -resize x2000 *

the new image size is perfect but the filesize increase over 3 times

0.png   -    1.8 MB  -   1461 x 2000
1.png   -    1.4 MB  -   1461 x 2000
2.png   -  666.3 kB  -   1461 x 2000
3.png   -  971.9 kB  -   1461 x 2000

what can I do to keep a reasonable size ?

P.S.: Tried to resize images with Gimp, 0.png reached to 3.4 MB!!! What is happening ?

Answer 1

It's likely that you were getting 16-bits per sample in your output PNGs. Use the "-depth 8" option to prevent that:

mogrify -resize x2000 -depth 8 *.png