If it was heavy, you knew there was an impressive manual inside, and if they took the time to make and print a manual, they probably took the time to develop the software well.
Here's Lotus 1-2-3: https://www.ebay.com/itm/283699892988
SimCity 3000 was perhaps the last game with a big printed manual: https://www.ebay.com/itm/354226256399
There were exceptions in both directions of course (for example, shareware-distributed Doom; random clip-art compilations with a crappy "editor" that came with a printed book showing each one), but if you were browsing a shelf it wasn't a bad rule of thumb.
Shortly after that everything went to small box with a CD/DVD in it (and a manual in PDF form, if any) and then even those mostly disappeared and now everything is online help.
It spawned the whole Missing Manual series, and there's something lost that interactive help can never replace - the manuals had a journey and a theme and you'd often find things you never even knew you wanted by reading them.
I still sometimes pick up old manuals at thrift stores just to read.