Thank you for getting in touch with us, we do appreciate your time.
Please may I ask if you may try setting your marker icons to “Retina ready” and confirm if this helps? You may do this by checking the “Retina Ready” checkbox when setting a custom marker icon.
Note that you will then also need to set a height and width value for the “Retina Icon Width” and “Retina Icon Height” settings (Maps -> Settings -> General) as the icons will then use the height and width values you have set.
With regards to your changing the size of the markers icons when zooming in and out of the map, unfortunately this is not supported.