From my manpage (on a MacOS 10.11 machine)
True if the file's size, rounded up, in 512-byte blocks is n. If
n is followed by a c, then the primary is true if the file's size
is n bytes (characters). Similarly if n is followed by a scale
indicator then the file's size is compared to n scaled as:
k kilobytes (1024 bytes)
M megabytes (1024 kilobytes)
G gigabytes (1024 megabytes)
T terabytes (1024 gigabytes)
P petabytes (1024 terabytes)
(suffixes other than
c being non-standard extensions).
So, since you didn't specify a suffix, your
-size 128 meant 128 blocks, or 64Kbytes that is only matched for files whose size was comprised in between 127*512+1 (65025) and 128*512 (65536) bytes.
You should use
-size 128c if you want files of exactly 128 bytes,
-size -128c for files of size strictly less than 128 bytes (0 to 127), and
-size +128c for files of size strictly greater than 128 bytes (129 bytes and above).