In 1.0.7. a change was made to allow symbols to preserve stroke widths when they were scaled to a different size. This was intended to be an opt-in setting for existing users for backward compatibility, but we got our logic slightly wrong and the setting is set for all users. There is also a further issue that is caused by this setting that means symbols that are grouped and then scaled by resizing the group end up in the wrong place.
If you are affected by this problem, or you would prefer the original behaviour where stroke widths remain constant when a symbol is resized, the solution is to open Preferences > Options, and uncheck 'Scaled symbols preserve stroke widths'. The setting applies to all currently open, new and future opened documents.
Apologies for any inconvenience.