I'm not sure about a direct comparison to the RM Nikki, but Ignes's Emilia hobo is absolutely huge. Stuffed full, it measures about 18" across, but because the default strap drop is also quite long (11" to the top edge of the bag; 13-14" when the top edge slouches down), you could definitely potentially stuff it and fill it with a tonne, IMHO.
The great thing about Ignes as well is that you can choose your own leathers and linings (fabric or suede), and customise it in other ways too (changing strap drops, adding or removing studs, mixing leathers in the design, etc). In terms of leathers, I'd definitely highly recommend a tinted leather, as those are super soft and thin, but also incredibly hardy. The tinted leathers will definitely give you a hobo that is very slouchy too. Check out the Ignes subforum for more info, there are some great ladies there who could definitely give you plenty of help and advice! (And the co-founder, Maria, who handles customer emails/orders, is wonderful to deal with too!)
Oh, another brand that does very large bags is Treesje. I think their Marley hobo might be larger than the Nikki (particularly in terms of depth), and if you get it in one of the softer leathers, it'll definitely be quite slouchy too. Stiffer leathers should still be okay too, as Treesje's leathers break in beautifully. They also have some other hobo-type designs that are deceptively large, which might work quite well for you! Some of these designs include the Haze hobo or one of the newer ones, the Exposed hobo. Some of these might also come in stiffer leathers (at least for the Haze; not sure about the new leathers for the Exposed yet), but as they break in gradually, they should definitely slouch more, IMHO.
How about a YSL Easy in Medium? It's definitely larger than a Nikki (about the size length and height when worn, larger width). I actually had to just sell mine off because it was too big for me and decided to keep a Nikki instead.