Hm, I may have hit on something. I found an old thread here on SSC from someone having a similar problem:

Thread

Seemingly, SQL Server 2005 (and maybe further versions?) can potentially pad the Log column with extra whitespace; in the above thread, the thread starter was having an entire gigabyte of whitespace padded into the table every day! I'd suggest running this:

SELECT LOG,LEN(Log)

FROM msdb.dbo.sysjobstepslogs

See if there's an unusually long LEN value for a field that seems quite short; if so, it's probably being whitespace-padded. I haven't seen much talk about the issue, but I'll dig around a bit more.