Here's my understanding -- someone please correct this if I am wrong:
If the majority of the cancer cells in a sample (more than 50%) are grade 4, the first figure is a 4.
If more than 5% of the cancer cells but less than 50% are grade 3, the second figure is a 3.
So in this case you would have a Gleason 4+3=7.
If the situation is reversed with the majority being 3, the Gleason is 3+4=7. This second case is not as serious as the first since most of the cancer cells are lower grade. But even so, the lesser volume of grade 4 is not enough to justify a lower Gleason.
If 95% or more of the cancer cells are grade 4, then the grade 4 is effectively occupying the 5% to 50% range as well as the "more than 50%" range. So in this case, you would have a Gleason 4+4=8. A greater volume of grade 4 -- therefore a higher Gleason.
Grades and percentages are judgment calls -- so the Gleason reported can vary slightly depending on who is examining the slides.
When it comes to cancer cells in the body, the actual volume of the original tumor is not a factor -- beyond the relative volumes taken account of by the Gleason score itself. The likelihood of an escape is much more closely related to Gleason grade rather than overall tumor volume. If it were otherwise, there would be some other grading system that gave greater weight to tumor volume.