First of all: The ZXing.Net is a different project/product, that has
common ancestry with ZXing, but is not a part of the base ZXing
project set. So it might pay to ask on their help channels than this
one.
Your 'problem' however is a more generic, so here it goes: the ZXing
generator generates a MATRIX, which is modules/pixels. Actual media
has a density, which comes into play. If your 1" circular label has
space for only 65 pixels, than it is a pretty bad media: the earliest
screens I remember had 75 ppi, 65 seems really low (~92 ppi if using
the diagonal). Most low-resolution print methods start at 150 ppi and
even the worst one I've seen in practice (an industrial thermal
printer) has 210 ppi.
The QR Code is not a free-size one: it has Version which specifies
the *EXACT* number of modules (a.k.a. pixels) the code must use [1].
The version also specifies the number of characters it encodes. So
depending on the size of the content you have to use a specific
version, and that is it. I have the feeling, that you're encoding some
text that needs version 6 (41x41), and all sizes between 41x41 and
82x82 will end up a 41x41 matrix with white padding to the matrix
size. If you want to fit that in a 65x65 that means the code must be
scaled 1.59 times, and 24 pixels will be doubled, while 17 will not,
this will produce s very distorted code, which will make it
unnecessarily hard to scan.
That is why scaling should be done from the minimal size using the
maximum available (reasonable) density of the print media/method.
Depending on your print method you might have access to smart scaling
that would provide extra density (like sub-pixel scaling).
I hope you can now understand the actual issue at hand: density of
the print media/method.
[1]
http://www.qrcode.com/en/about/version.html
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