My dogs are around the 6KG mark too. Big dogs in little packages, though LOL I hear what you're saying about perhaps feeling you're not giving enough. Either grin and bear it (my dogs certainly didn't complain at the apparently reduced rations), or else cut the feeding amounts and add a raw meaty bone, raw green tripe or some raw mince (containing ground bone) or something.
NatureDiet (which I confess I had to look up) isn't that impressive sounding, considering. I picked the Puppy one as it'd be the most likely to have high animal protein and less filler (grain, veg).
That ingredients list doesn't add up to 100% (more like 73%), so where's the other ~27%? Notice the "MINIMUM..." next to the meat, rice and the veg? I'm willing to bet an extra 27% veg or rice is way cheaper than an extra 27% whole cuts of fresh meat... If it was meat they'd be shouting it from the rooftops, surely?
It's 48% protein and 36% fat so not too bad at all on that score. Note, though, that Orijen is already 70% meat and WAY more attractive than 'crap in a bag' brand dog food, those high in grain etc. My dogs have literally been grown, born and raised on raw food for generations, no kibble involved (except odd exceptions). Even they went mental for Orijen when they smelt it, and they will normally turn their nose up at any commercial food, not even recognising it as 'food' lol
You can also always add a little hot water to Orijen. It doesn't swell up like grain based kibble does and will likely become more attractive (heat + water = smell increase!). But as I said you'd be fine (even better off in a way) to supplement with some raw meat, green tripe or suchlike if you prefer. Feed whole LARGE raw meaty bones at least once or twice a week, more if possible. This will help keep the teeth clean, and the oral cavity free of disease. It'd work out cheaper than NatureDiet too.
In preference to feeding NatureDiet with the Orijen, you could try
400g Frozen Minces - The Natural Food for Healthy Animals from Prize Choice / AMP. They contain about 5% ground bone (for calcium) so they're more balanced than plain mince, and you can get a decent range of types to try out. They're cheap too (about 33% cheaper than NatureDiet), and being raw, more nutritious. Just pop a block in the fridge at night, and it'll be defrosted ready to feed the day after (and will "keep" another day again for the day after, but be sure to have a second block defrosting if you feed that way, so you don't run 'dry'!).
If they're not used to such things (raw meats etc) I'd get them settled on Orijen before I started throwing even more new stuff at them though. As an interesting aside, perhaps it's because my dogs are raw fed to begin with but I noticed tons of sloppy faeces when I tried Wainwright's for a week.
An immediate 'cold turkey' (pardon the pun) switch to Orijen and their motions were small and rock solid again the day after and ever since. I never did get the whole 'omg swap so slowly you can barely tell' thing.
Maybe raw fed dogs just have tougher constitutions

