I’m confused

Pixoto upload appears soft and lacking in detail. Is this a feature to get better image compression if you pay?

I have noticed for a while now that the pictures I upload appear soft and detail is lost. The latest is this picture. I uploaded on 500px , and facebook and all the detail is there. When uploaded on pixoto, the neck area is soft. Is there a particular size i need to upload or do I need to oversharpen for pixoto? https://www.pixoto.com/images-photography/animals/birds/copper-rumped-humminbird-6521612526026752 compared to https://web.500px.com/photo/1015822233/Copper-Rumped-Hummingbird-by-Edison-Pargass/
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