Blu ray - Dual layer - compressor

Go into Compressor and choose the H.264 for Blu-ray template, then in the Inspector window switch to the Encoder settings and toggle the File Format from H.264 for Blu-ray to anything else and then back again. When you do that then you'll see the Stream Usage control is now enabled and set to Blu-ray (before it was locked to AVCHD) also, with this setting you'll see that the Bit Rate controls now allow you to push up to (Avg) 30 Mbps and (Max) 35 Mbps respectively. Go ahead and push those up, then click the "Save As" button at bottom right and give your new custom preset an appropriate name. Try that one and see if it gets you what you want.

90 minutes x 60 seconds x 30mbps / 8bits per byte = 20.25 gigabytes. This will fit on a single-layer disc, and is the max size you can make a 90 minute movie.

Why does it matter how big your timeline is? If 30mbps is the max bitrate, then it is the max bitrate. BD supports VC1, H.264, and mpeg2; it does not support whatever format your timeline is.

The source's filesize is determined by the source codec's bitrate and the source file's length : source bitrate * source length.

The target's filesize is determined by the target codec's bitrate and the target file's length : target bitrate * target length.

Even though your source and target length are the same (in this case) the bitrates are still completely different ... so you see that you your source's filesize has absolutely nothing do with the target filesze but rather has everything to do with bitrate.

And the limit here is Compressor's built in options only go as high as 35Mbps max. If you want to encode at a higher bitrate then you need to either use a different compression app which natively supports higher bitrate encodings or you can augment Compressor with 3rd party hardware or software that allows higher bitrate encodings. For example, I can use my Matrox "MAX" equipped MXO2 (I/O box) to create accelerated H264/Blu-ray encodings, in Compressor, up to a maximum 50 Mbps.