You can scale-up or scale-down the chart before exporting using exportChart method and reset it back soon after exporting. Please take a look at this JSFiddle for a working example.
If the solution suggested doesn’t fulfill your requirement, kindly create a sample project reproducing the issue you are facing and share it with us over Google-Drive or Onedrive so that we can look into the code, understand the scenario better and help you out.
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Thangaraj Raman
Team CanvasJS