The total size of the output bam file has decreased

[QianZixi]

The total size of the bam file outputted by methphaser has decreased.
My bam size after Whatshap processing is 175 G, but the total size of the bam after methphaser processing is 142 G.
175G Nov 30 01:33 HG002_ONT_md_sup.whatshap.haplotag.bam

total 142G
 6.7G Dec  1 15:20 output_ob.10.methtagged.bam
 6.8G Dec  1 15:47 output_ob.11.methtagged.bam
 6.8G Dec  1 15:28 output_ob.12.methtagged.bam
 4.8G Dec  1 15:25 output_ob.13.methtagged.bam
 4.6G Dec  1 15:53 output_ob.14.methtagged.bam
 4.2G Dec  1 14:55 output_ob.15.methtagged.bam
 4.4G Dec  1 14:58 output_ob.16.methtagged.bam
 4.2G Dec  1 14:56 output_ob.17.methtagged.bam
 3.8G Dec  1 15:23 output_ob.18.methtagged.bam
 3.0G Dec  1 14:42 output_ob.19.methtagged.bam
  12G Dec  1 16:02 output_ob.1.methtagged.bam
 3.1G Dec  1 14:44 output_ob.20.methtagged.bam
 2.0G Dec  1 14:38 output_ob.21.methtagged.bam
 1.9G Dec  1 15:20 output_ob.22.methtagged.bam
  12G Dec  1 15:59 output_ob.2.methtagged.bam
 9.9G Dec  1 15:54 output_ob.3.methtagged.bam
 9.4G Dec  1 15:52 output_ob.4.methtagged.bam
 9.0G Dec  1 15:44 output_ob.5.methtagged.bam
 8.6G Dec  1 15:42 output_ob.6.methtagged.bam
 7.9G Dec  1 15:52 output_ob.7.methtagged.bam
 7.3G Dec  1 15:37 output_ob.8.methtagged.bam
 5.9G Dec  1 15:16 output_ob.9.methtagged.bam
 3.9G Dec  1 15:10 output_ob.X.methtagged.bam
 489M Dec  1 15:27 output_ob.Y.methtagged.bam

This is how i run mathphaser :

meth_phaser_parallel -b HG002_ONT_md_sup.whatshap.haplotag.bam -r hs37d5.fa -g whatshap_hg02.gtf -vc whatshap_hg02.vcf.gz -o work -t 16
meth_phaser_post_processing -ib HG002_ONT_md_sup.whatshap.haplotag.bam -if work/ -ov output.vcf -ob output_ob -vc whatshap_hg02.vcf.gz -t 16

What is the reason of the problem?How can i solve it?
Thanks!