Encoding for Encore from Premiere Pro - File Size

Question

I'm working on a Blu-ray project that contains four separate tutorials, each with several individual clips.  I exported each clip (chapter) out of After Effects CS6 using the standard H264 preset.  The tutorials range from 2 to 5 hours each.  After all was said and done, the total file size of all H264 video clips was just under 25GB, perfect for a Blu-ray burn.
I created four sequences in Premiere Pro, one for each tutorial.  All of the H264 video files were brought into Premiere Pro, chapter markers were added, ready to rock.
When I attempted to export each sequence using the H264 Blu-ray preset, I found that using that preset would result in a grand total of about 75GB worth of data.  So I selected the MPEG2 Blu-ray preset, and it was almost the same.  I manually adjusted target bitrate settings, dropped quality to as low as it would go, and still the estimated file size far exceeded the 25GB I had available.
All tutorials total about 15 hours of 1280x720 30p content.  I realize that's a ton of data, but AE exported all of this content using H264 and the grand total file size was under 25GB.  But bringing those very same video clips into Premiere Pro, assembling them, and exporting them results in much, much larger files.  I even attempted to import the AE exports directly into Encore to avoid having to re-encode anything, and still it estimated that the project's file size exceeded 75GB, even though the source footage of H264 video was less than 25GB.
Right now I'm down-rezzing all of the footage to 720x480 and the file sizes are smaller, although I'll still need to decrease the bit rates significant if I have any shot of dumping all of this onto a single 25GB Blu-ray.
I do realize that 15 hours is a lot for a single disc, but what I don't understand is how I can have 15 hours of H264 exported out of AE result in less than 25GB, but using that exact same footage in PPro/Encore results in over 75GB of footage.
Any insight would be appreciated.
Windows 7 64-bit
Adobe Creative Suite Production Premium CS6
(After Effects/Premiere Pro/Encore)

Answer 1

The thing is, the AE exports look just fine.  (The tutorials are just a bunch of screen capture/software stuff.)  I didn't mess with any of the default preset settings for H264 out of AE, but yeah, they must've been encoded at pretty low bitrates.
I wasn't aware that BD specifications have minimum bitrate requirements, so that would explain why I'm unable to decrease the bitrates within PPro or Encore.  Thanks for the info on that.  I just figured as long as the footage was at least 720P with square pixel ratio encoded in H264 or MPEG2 I'd be fine, but apparently not!
The tutorials are for my own personal use, so I'm not too concerned about decreased resolution if I am able to make the SD content fit.  Otherwise, yeah, burning multiple discs is my only option.
Thanks for the prompt feedback.