Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
I have encountered the "memory allocation failed" problem. I'm trying to convert a 472MB folder with TIFFs to a PDF file.
I use current version of IM, specifically ImageMagick-7.0.2-7-Q8-x86-dll.exe on Windows 10. My GhostScript is also current, that is, Ghostscript 9.19 (2016-03-23). To be sure that everything is OK, I unistalled IM and GS and installed once again GS and IM (in this order).
The size in bytes of the TIFFs is irrelevant. How many TIFF files? How many pixels per TIFF? Are they colour or grayscale?
How much memory on your computer? What operating system? (Windows?)
It might be helpful if you gave the exact error message. Although it should put pixel caches on disk if there's not enough memory, it allocates smaller structures in memory, and gives up if there isn't enough free memory.
As a workaround, you might try compressing each TIFF as Group4 (in separate commands) before combining all these into a single PDF.
4209*6022*75 = 1.9 G pixels. In v6, Q16, these would need 8 bytes/pixel, so 16 GB. V7 needs (I think) only 2 bytes/pixel for grayscale, but that is 4 GB. So you run out of memory. If pixels won't fit in memory, they should be put to disk. In your case, you may have used all the memory with pixels, then tried to allocate something else.
To force more pixels to disk, edit policy.xml. I think you should uncomment "memory" and "map" and set them to 1GiB or 500MiB or whatever.