[Dx-qsl] Number 9 envelope OK for return envelope in Europe?

[Tim Goeppinger]

I wish to be as kind to the European QSL managers as possible, which is
why I am asking this question.

I usually send a Number 9 (98.4mm × 225.4mm) for my return envelope.
A couple of my European QSLs have arrived with my Number 9 envelope
modified so that the length was shortened to make it more like a C6 envelope.
My latest from H4ØFY arrived in an envelope that is slightly smaller
than a C6 (he discarded my Number 9).

Is there a higher Air Mail rate for envelopes larger than a C6 in some 
countries?

Thanks & 73,

Tim K6GEP