Ok, first, off the bat, 778 Mb is not a normal size for a SU model. you’re on the high side.
right now my computer is trying to open it, but you have to keep in mind, the heavier a file, the higher the risk of having a writing error in it. In any software.
Think of it as text. if you write a single sentence, you’ll make fewer spelling mistakes than if I ask you to retype moby dick.
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it’s open, and SU is using 70% of my RAM and extra ssd space just to open. Chance is, your computer either doesn’t have enough free RAM, or won’t allocate extra SSD space to help the RAM.
I checked the Tags, then purged. As I opend it, my style panel was open, and I could see you had 20+ styles in it, it told me you needed purging (simply go to model infos, statistics tab, and purge)
For this model there is a lot of incorrect tag usage. ALL edges and faces should be created and remain untagged. (yeah Dave, I just copied your sentence from another thread. This sentence should be part of the result window, below the count. 
)
I’m purging right now, for the past 40 minutes, then I’ll check material resizer, and I’ll come back here to reply with a lighter file ok ?
purge took a good 40 min, but as I’m not the sharpest spoon in the drawer (it’s sunday morning), I accidentally closed the result window without looking / screenshoting. Can’t tell you how exactly many things it removed.
But I removed a lot. 815 to 430 Mb is a lot.
I also removed the profiles from your style, it’s not exactly weight reduction, but it makes navigating way easier.
Then material resizer tells me you have a LOT of materials that are simply huge.
the material for a simple leaf shouldn’t be 2000 x 3000 px. or a wood table top 4000 x 5000. and I’m pretty sure some of those could actually just be plain colours.
I arbitrarily choose to reduce all materials to 1024px. Material resizer is a free extension, feel free to use it from time to time, after you downloaded some 3d warehouse files for example, they tend to bloat quickly.
Reducing the materials took another 20 min, 69 materials for 60 Mb. I’m now down to 369 Mb of the 815 initial.
that’s a 55% reduction.
Your situation is quite common, and the main culprit is 3d warehouse. and the perso who showed you how to use it. The warehouse is filled with high resolution high texture models. And they are gorgeous. However, they add up. A 5mb couch isn’t much, but with a 5mb plant, 5bm door and 5mb lamp you get a 20mb set of 4 objects.
Many objects have a ridiculous level of details, carpets or books. you need to find a balance between the “it looks good” factor and the “oh my god my computer is burning” one.
A tool like material resizer can help you regulate the weight of the stuff you download.
And as a good practice, purge. In the component panel, you have a small menu (top right) that allows you to purge, ie. removing all the components that you decided not to use. that’s how I gained 400 Mb, all of the things you didn’t use but were still there.
Some of the model you’re using have been modelled 10x bigger, to have maximum details, but at actual scale, we end up with this nightmare of a geometry
So here is your file, after 1h30 of deep cleanup. I took the liberty of removing profiles from the style, as well as putting ALL of your roof / ceiling elements on a ROOF tag, to make it easier to access from the top.
Your file needs a lot of cleaning up, I see multiple accounts of a components containing only a component containing only a component containing raw geometry, that’s 2 components too many.
At some point you’ve lost control of this file, and it should be easier now to get it back, but you need a safer approach in your design, lower resolution objects, and a healty purge / material cleanup from time to time 
Because 381 Mb for a pair of houses is still way too much imo.
and now you should be able to open it.
https://fromsmash.com/dJRi.Z2pLk-ct
(link will be good for a week)