Ok lets make an effort to educate you on reality a little bit. Victorian era was an era when colonizing nations had advantage over colonized ones, have you heard about that? So in perhaps 95 percent cases small expeditionary force, just as advocated in this post, was enough to achieve goals. If you need a proof for that, go and read a book. I can recall from my memory several cases, however, when colonizing nation was forced out without achieving main objectives and didn't simply use your brilliant idea to send 300k troops, I wonder why

1. Mentioned Adua.
2. First british invasion of Afghanistan.
3. Failed russian expeditions to Central Asia, most notably Bukhara.
4. French war with china.
5. French intervention in Mexico.
6. First british operation against Muhammad Ali.
7. Spanish attempts to retake Mexico.
8. Russian japanese war - perhaps russia should just send another 500k men from other parts of empire just like Britain in your amazing example?

9. Khivan campaign of 1839.
10. French expedition to Korea.
11. Egyptian invasion of Ethiopia (Egypt as more modernized power)
12. And hey, first boer war, why didin't british send mighty 300k at that time instead of making peace, I wonder

13. Basuto Gun war.
14. Second Mandingo war
And so forth. Literally EVERY single one of this war could end differently if GP sent more troops. I wasted a fair amount of energy in my fingers to try to explain to you that not every colonizer can send 300k men to die in jungle. I hope you will benefit from this 
PS: which doesn't mean that in SOME very special cases a GP can't send more forces ofc, but it's SOME, in other words, exception.