Dominik Dzienia - 2015-02-04

Size (parameter) means more like pixel size multiplier, exact code size (aka Version) is determined by lib on the fly.
Also - lib tries to switch internal encoding (from Numerical, ALphanumerical, 8-bit) on the so it explains why some shorter text may have bigger codes - their encoded bitstreams are different, i mean:

12345
ABCDE
&P-*+

all that may (and should) have different sizes, as they are encoded differently
another thing to check is - if that algorithm do well on switching contexts