From the sounds of it you already have Sandy's in large so why not just try those? I usually did that at nighttime for my kids, put them in a Sandy's one size larger than they wore during the day (eg, I'd put them in larges when they wore smalls and Toddle-Ease when they wore larges). Usually there's no poop to deal with at night so it's not as important that the diaper fit snuggly around the legs so even if it's too big it's not as big a deal...
The One-Size would probably work too. They are more absorbant than the small Sandy's (although less absorbant than the larges) so that little extra might help. They're not trimmer than the small Sandy's but with the fold-over at least some of the bulk would be spread differently so it might be less bulky overall than a Sandy's with 3 liners.
Another thought would be using bamboo liners if you're not already. They're the same absorbancy as cotton liners, but trimmer... Or if you have the large Sandy's liners maybe you could use one of those (either 2 larges or 1 large and 1 small) and fold them strategically so you'd have slightly more absorbancy than 2 liners but maybe not as much bulk as 3 liners. You could also maybe experiment with putting a liner or two on the *outside* of the diaper instead of inside (still inside the cover of course, just lay it on the cover itself before fastening - it'll probably shift around a little but it'll stay inside so it'll still provide the same absorbancy). Same absorbancy but being outside the diaper might make it look/feel a little less extreme.
Remember also that really as long as it doesn't leak, it's pretty normal for nighttime diapers to be pretty soaked in the morning. That said, I understand the desire to not have to deal with a sopping wet morning diaper - I used to put more liners than was strictly necessary just to avoid it. Also remember that the bulky nighttime diaper probably bothers you way more than it does her. Most kids just really don't care, especially when they're sleeping all night anyway (well, ok, maybe not sleeping per say but you know what I mean, LOL!). For a while I even double diapered my son (if I remember correctly it was a Toddle-Ease diaper over top of a lg Sandy's). Now THAT was bulky! But it worked and really my son just didn't care...