It isn't clear how you have 'turned down the resolution'. Are you using proxies for this or simply lowering the comp's or layer's resolution and quality setting respectively?
Another thing to nore is that using the QT PNG isn't something most professionals use. For Alpha Channel support, such users go with either uncompressed or Animation or a CODEC that is native to their video I/O.
You will want to provide info on the resolution of the source footage and details on how you lowered the resolution of the source footage.
Cheers
RoRK
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