SQL Server 2012 Index Reorganize Logfile Growth
In full recovery model, internal log truncation depends on you taking a log backup. If you wait three hours in between them, you'll have three hours of transactions sitting in there. 

Index maintenance, as you've noted, is fully logged. If you truly perform some maintenance on every index in your database (or even most of them -- it's hard to say without knowing at what threshold you reorganize indexes), it's entirely possible for your log file to need to accommodate every one of those in between log backups.

Let's say in the 30 GB database you reorganize indexes at 5% fragmentation (a common tactic), and most of them have hit that level. You'll need to perform some action on all of those. In the other databases, if you have the same threshold, they may have far fewer indexes to touch. This could explain the discrepancy. 

I understand that not much is under your control here, but if I had my DBA druthers, I'd want to


Take more frequent log backups
Perform less aggressive maintenance
Use Ola Hallengren's free scripts


Using Ola's scripts is a no-brainer. You can set very specific levels to reorg or rebuild, which indexes to skip or include, and at what size you want to perform some activity. I get that this isn't your question, but should you choose to share this with management, I'd be a bad person if I didn't include it.

The other plus side? All the actions that Ola's scripts run get logged to a table, so you can look at what actually happened. That's the most important thing to have around for forensics if you wanna find out why something happened.

Hope this helps!