The first MiniSEED file might be smaller for a couple of reasons
(reading and writing MiniSEED with ObsPy should not change the record
length nor the encoding):
(i) The records might have been padded due to whatever reasons. ObsPy
would have removed the padding.
(ii) There might have been some blockettes and using `obspy.core` to
read and write MiniSEED does not retain all blockettes.
(iii) Maybe there were some noise/shadow records in the data? ObsPy also
removes those.
The second MiniSEED file is likely larger because the instrument
deconvolution converted the data to double precision floats. Otherwise
MiniSEED and SAC should be very similar in size as neither compresses
floating point data and the difference is only in headers and the fact
that MiniSEED is bound to its record size.
You can convert your data to single precision before writing it which
should result in a smaller MiniSEED file:
import numpy as np
for tr in st:
tr.data = np.require(tr.data, dtype=np.float32)
Cheers!
Lion