When using bitmap files like PNG, it is always better to have a specialised program handle sizing. Though browsers have been much improved in this regard throughout the years, they are not as heavily optimised as specialised image handling programs such as Photoshop or GIMP.
Photoshop has a batch image resizer, which you can find under File > Scripts > Image processor.
GIMP does not have a handy out-of-the-box tool, but has some options. If you're not afraid of scripting, you can use the Command Line Interface. There are also a few plugins that provide a Use Interface, such as BIMP or David's Batch Processor.
ImageMagick is a command line tool that can handle all kinds of image manipulation, including batch resizing. You can find a tutorial here.