PNG size become bigger when after thumbnail the png

[needkane]

See http://www.imagemagick.org/script/architecture.php

Q8 uses half the memory of Q16.

Image ：http://s3-us-west-2.amazonaws.com/qiniu-bs/2.png
Type ：Palette2

If use command:
convert 2.png -thumbnail 400x b.png //size bigger than origin

But use command like you :
convert 2.png -thumbnail 400x -colors 2 -type Palette b.png
or
convert 2.png -thumbnail 400x -colors 1 -type Palette b.png

The result only one color and Opaque

[needkane]

See http://www.imagemagick.org/script/architecture.php

Q8 uses half the memory of Q16.

Image ：http://s3-us-west-2.amazonaws.com/qiniu-bs/2.png
Type ：Palette2

If use command:
convert 2.png -thumbnail 400x b.png //size bigger than origin

But use command like you :
convert 2.png -thumbnail 400x -colors 2 -type Palette b.png
or
convert 2.png -thumbnail 400x -colors 1 -type Palette b.png

The result only one color and Opaque

thank you again,i know the answer
-alpha on