While Magneto Cumulative Generators (MCGs) differ widely in size and operating regimes, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that would be solely based on physical principles and manage without heuristic factors. This means, all programs known to the authors utilize an additional parameter that adjusts for the loss in flux that is currently unexplained. Experimental efforts revealed that the unexplained loss in flux becomes smaller the larger the generator volume is. Specifically, for generators with constant diameter helix and armature, the figure of merit, β, for generator performance exhibits a distinct decline with the angular frequency, ω, of the progressing contact between end-initiated armature and helix. Since ω is proportional to the square root of the ratio of initial MCG inductance to compressed volume, it becomes apparent that the generator performance is limited by size. For large generators on a meter scale, a β of approximately 0.95 has been reported (β = 1 means no flux loss at all), and own tests indicate that β drops quickly to about 0.7 to 0.6 for generator on the 10 cm scale. Still, even very small generators with about 30 mm outer diameter can have a β of around 0.6. Tapered generators, with tapered helix or tapered armature might perform differently and can be used to further assess the situation. Preliminary results for MCGs with a tapered helix show a performance that is somewhat superior to the straight helix generator.