Well, as you have already found out, the things actually WERE made. They were tried out in the Trenches, which was static warfare at ts finest, and found wanting even under those conditions (nice firing-step to work from, sandbagged positions and so forth). The thing added a POUND to the rifle and made shooting from the prone position (preferred in the British Army because it is MORE ACCURATE) nearly impossible. The extra depth made the rifle clumsy for bayonet work and, when you have somebody coming at you with 20-1/2 inches of Seitengewehr 98, you do NOT want to be clumsy.
Besides, to what point? ANY rifle which gets good and hot starts spraying its bullets.... and there are few ways to get a Lee-Enfield HOT faster than a 20-round burst. The rifle has to cool SOME time..... unless you want to set it on fire.
Yes, there were battles in which a lot of ammo was expended very rapidly. Second Ypres in the St. Julien sector comes to mind: 1 Company of Winnipeg Rifles held off 3 advancing German DIVISIONS that morning. They fired the rifles until they were too hot to touch to reload them, changed rifles with a dead man and kept on until THAT one was too hot to touch. And they did it with Rosses, which were/are greased lightning to load and fire but which have only a 5-round mag. At least the poor things had time to cool... a bit. Accuracy was not a great factor in that particular fight; it was described to me by one of the participants as "too close to miss".
Jerry tried the same thing, making up 10- and 20-round magazine EXTENSIONS for the 98 Mauser. The box will fit the Kar 98aZ and the original Gew 98; doubtless it would fit an original Kar98 if you were rich enough to afford one and lucky enough to find one. These were not magazines, they were extensions, using the magazine box already in the rifle and extending it to 15 and 25 rounds respectively.
I have an original German magazine extension, made by Bing of Nuremberg, the toy manufacturer. I have actually tried this. You start with a nice lightweight Karabiner 98aZ and remove the 4 ounces of magazine guts and put them in your pocket. Now you add a Sling (most of a pound) and this great ugly magazine extension (another pound) and you add a 1904-type Pioneer bayonet for another pound and a half. NOW you strip in 15 rounds from Chargers and insert the final 10 by hand (because that spring is getting DAMNED stiff). There: you're ready. Now try running with the thing. When that gets old, try dropping down onto your belly like a reptile and banging off 10 fast AIMED rounds.
Believe me, you will WISH for a nice, reliable Chauchat! A full-wood Garand with all the trimmings is a LIGHTWEIGHT in comparison and, just because the Mauser is heavier, it has no gas action as the Garand has...... so it keeps on kicking you black and blue.
The large-capacity mags which the British made were withdrawn about the same time as Fritz withdrew the magazine extensions, and for the same reason: they were IMPRACTICAL in a Real World situation. A FEW of the German magazines were still in use as late as September of 1918 on the Russian Front, but they had been withdrawn more than a year previously in the West. I got that from a man who was in both places in that time frame.
That said I do know that there was a single Jungle carbine once made up by a fellow named Don Turner in Vancouver, about 45 years ago. Don modded the JC to use a single-piece stock, a Bren Gun magazine (permanent mods to the triggerguard assembly and to the Body of the rifle) and a Bren Bipod. He also had the thing set up to use a scope. It was supposed to be his rifle for SHTF but the S hit Don instead; he is gone now these many years and I have no idea what happened to his JC. It was completely useless, anyway: just imagine a 12-pound Jungle Carbine which can be shot only from a high rest!
But the big mags, both types, were impractical.
Besides, if you have CHARGERS and know how to use the things, BOTH rifles can be loaded faster with the Charges than if you were to switch magazines.
Hope this helps.