Ping size is checked before compression https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/TelemetrySend.jsm#1160. It seems that it is better to check TelemetryStorage.MAXIMUM_PING_SIZE after compression.
This is a good point; the size after compression is what will actually be meaningful for upload success etc. We should probably look into this after we have an understanding (and monitoring) of ping size vs. upload success via bug 1318297 and bug 1367094.
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