Yep exactly - to give you an idea, looking at my plans the internal walls are 70-90mm, and external are 160-250mm. All adds up!
Don't worry so much about those two length and width measurements - unless you're building an exact rectangle or square, I mainly used it to compare with block dimensions.
Like Gogo65 said earlier, 7 bedrooms and 4 bathrooms into less than 350sqm - there's a lot going on there
In contrast, mine will be about 350sqm when all is said and done and it's only 4 bed and ~3 bath... If I converted some of mine into a 2 bed granny flat then for sure space would have to be taken from all over the place. It's just the nature of what you're trying to achieve.
Cutting the GF out, the house looks about 267sqm, which for 5 bed and 4 living isn't a monstrous house by today's standards, but still a good size.
For me, the best thing to do with this plan would be to maximise the spaces (no wasted nooks or anything), try to open up as much as you can, get some nice high ceilings, and protect from that western sun. Could it be flipped so the granny flat is on the western side? Harsh, but would mean you could throw windows at the main house all over the place.
Btw, you'll feel the garage is small... it's the same size as my current garage, and my two large sedans don't leave much room for storage. Where you can, try and compare dimensions to your current house (with a tape measure or string) to get a sense of perspective.