I thin I've seen this somewhere on your blog, but I can't find it now. "Mwen we ou fe yon ti desann" would mean "I see you lose weight", right?

Yes, that's what it is.
I don't remember talking about desann.  I do remember talking about kase.  Both can mean to lose weight.

Mwen wè ou fè yon ti desann.
I noticed you've lost some weight.

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