This is a personal view, there is no absolute truth imho. So it's hints and clues, nothing more.

The first time I held a Sony Alpha 7 body in my hands, I was delighted by it's small size. The difference to a full format DSLR is striking. Apart from my large format b&w studio work, I'm into outdoor photography and I love wide angles. I run around a lot, weight and size are an item. Up to now this was mid format cameras, folders (no wide angles) and 24x36 (SLR and M39 rangefinders). And growing difficulties to get film develloped.

The A7 seemed a good solution for me. The first disappointment: size and price for Sony mount full format lenses. So the so-called legacy lenses and their adapters came into focus (pun intended). I don't mind to focus manually, I have done so all my life.

Some data from the Sony website: the A7 camera: 126.9 x 94.4 x 48.2 mm) (W/H/D), 474g, with battery and memory. This is really small and lightweight. Then some Sony lenses: the famous 55mm lens: 64.4 x 70.5 mm, 281g; 28-70mm kit lens:72.5 x 83 mm, 295g; 24-70mm Zeiss lens: 73 x 94.5 mm, 426g; and the 70-200mm lens: 80 x 175mm, 840g. So the first 2 weigh about 2/3 of the camera weight, the Zeiss lens nearly as much as the camera and the tele nearly twice as much. They are all big compared to the camera, with the tele lens you hold a big lens with a small camera attached. Weird.

Whether you buy old Olympus, Canon or Minolta lenses, is a personal choice. Olympus are usually a bit smaller. I like them well. If you look at the sales on internet, they seem to be more prone to fungus than the other two. M42 are often a bit cheaper and there is an enormous choice. Russian lenses are a temptation. At the end of WW II the Russians came into posession of what the Americans had left from the Zeiss factories. So there is a Russian equivalent of nearly all famous Zeiss design lenses. Most legacy lenses are smaller than the Sony lenses and weigh less, adapter included. I lately came across another compact solution:
Konica AR. The lenses are 40.5mm FFD. This is shorter than most of the others, except M39. So the adapters are shorter as well. On top of this, AR lenses are very compact, modern and luminous.

Buying lenses on internet is not easy. I bought more than a hundred for this project. Fungus is a real problem. About 30% of the lenses had fungus and another 20% had severe dust or heavy scratches. I am not talking about a spec or two. So about 50% of the lenses had real problems, visible with a short glimpse through the lens. And I only bought from sellers with a good reputation. This is a lot. So I nearly stopped buying via internet, I keep my eyes open on fairs and flea markets. There you can touch, feel and have a look yourself.

As size and weight are an issue for me, the
so-called M39 lenses are my preferred solution (in fact, it's not M39 exactly, the diameter is 39mm, but the pitch is 0.977 instead of 1, so LTM or LSM  Leica Thread (or Screw) Mount is more exact). Have a look at the A7 lens comparison page and you will see why I prefer them. No problem to carry around the A7 with one M39 lens attached and to have two more lenses in your pockets. + 2 or more batteries, battery life is poor (or a shame if you prefer). However it improves significantly after some loading cycles. If you compose your pictures with care and focus manually, a full battery will only give you about 150 photos. The battery grip is absolutely no solution to my eyes, it's big, expensive and only doubles capacity. A really poor solution. It's easier to carry some spare batteries.

I love my A7 well. The results seem to be better than those with classical 135 film cameras. The possibilty to have special lenses for each situation at very low expenses is real fun. Which lens is best depends on your needs. Please compare the sample pictures to find out.

For the A6000
I chose the smaller lenses for an Alpha 6000 comparison page. I had to learn about diffraction and APS-C size sensors, a bit of a deception. I hadn't read about it before. But now I know and you may, if you refer to the A6000 comparison page.

That's it for the moment. One last remark about my view of photography: There is nothing worse than missing a photo. A non-optimal one is still better than none. So I always carry a tiny camera in my pocket. I started with a Minox 35 in 1975 which was replaced by an Olympus XA in 1979. It stayed in my pocket for 30 years. I tried small digital ones in the 2000's, but it was only in 2009 when I came across the Canon S90 that I found one that was at least as good as the Olympus XA in my eyes. These tiny little electronic wonders are completely worn after 3 years (and about 15.000 photos), so I am at my third, a Canon S120. At the end of the camparison pages there's a link to my test of it. I still like it, even if picture quality of the Sony A7 or the A6000 is much better. But they don't fit into a pocket. At least not yet.