Minor size differences in output bam file

[chasingdex]

Hi @FelixKrueger I have three bam outputs generated with identical parameters and identical versions of Bismark and bowtie2 using a set of paired end trimmed inputs. Three parallel parameters were used- 8, 12 and 16. The total no. of reads aligned to the genome according to their respective alignment reports are the same.

However, the bam files have a size difference. See below. What could cause these size differences?

• Parallel thread; Bam size
• 16; 86370183058
• 12; 86250282377
• 8; 86111223110

[FelixKrueger]

This. is probably a question better directed towards some compression afficionados - data simply compresses slightly different depending on how you do it, or in your case - which reads come first. I have seen size differences of nearly 2-fold with (read name) sorted BAM files for the same information content...