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 > ...
You actually might be interested in a brand new feature only available on the latest Sketch Beta (Sketch 58) as of today, September 1st.
Here's the link to that beta: https://www.sketch.com/beta/
As you can see :
Introducing Smart Layout With Smart Layout, we’re making Symbols more powerful and supercharging your workflow when it comes to using them in ...
The method above seems to be a bit touchy on complex images with a lot of layers.
I don't think it works for text but if you convert text to paths it should work.
Any grouped layers have to be found and ungrouped and I didn't have any luck with selecting all objects in all layers and ungrouping. I think you have to actually get the grouped objects.