Why do you even need to delete it?.. Just drop it on the ground, sell it, put it in a barrel or on the body of dead guy. "here dead guy.. take this crappy knife I don't want."

As multiple people have already stated, save game bloat.

When you drop something the game has to store what cell id the item is in as well as x,y,and z coordinates it is located at for that cell.

For bodies it has to store the reference id for what body it is stored in.

Ditto for containers that items are put into.

Do this enough times over a 200 hour playthrough and you've got a save file that may be hundreds of megabytes large. It wasn't unheard of for loads of such saves to take minutes to process, sometimes even crash in the process.

As a rule in these games if you've touched it the game saves where you moved it so that it is in the same spot next time you load the game. There are only a few respawning interiors that reset item locations. Think the kitchens of whiterun's keep in skyrim. Any dungeons though don't reset item locations AFAIK

Putting it in a container (or a body...a unique container) does NOT bloat your save (the container is already in the cell and the list of what is in the container is very little data compared to storing the positional data if you drop it on the ground).

Dropping it on the ground DOES bloat the save (save game has to keep up with the object added and the positional data of where it is now), so that's not the best thing to do.

Keep in mind, that the changes they have made to the FO4 engine may make this not be the case, but the object positional info detailed above is pretty low level stuff and it's use has kind of defined that you can potentially roll a thousand wheels of cheese down a hill or pile a thousand tin cans in a room in Beth games. I seriously doubt that has changed much (tho they did some nice work in cleaning up dead bodies from Skyrim cells with some later updates).

Was save game bloat a big problem for some people? My 1st save game for skyrim is 4mb.. my last save game is 14mb.. and that's an over 400 hour game with a crapton of mods. Doesn't seem too bloaty to me.

Was never a concern for me. Some people seem to think that it contributes to corrupted files or slow loading, but I never had any problems with save files in BGS games, personally.

Skyrim resets cells to save on memory. Annoyingly it does it with decorations in your house.

On PC yes, but on consoles those MBs add up.

Bloat is bloat, and I doubt their size differences are as much as you perceive them. I'm a programmer by trade.

The most efficient save method they could be deploying would be a global map structure with reference ID keys and a locational base class structure associated to each key, which could either be polymorphed into xyz/cell id objects or a reference id to another container that it is inside of.

Its the difference between another entry in a map data structure for a container or body (likely two integers) and 3 floating point values for position and an integer for cell id.

In practice integers and floating point values occupy the same amount of space unless you're using really high precision floating point variables. (Which isn't necessary for a game.)

So unless there is some cockamamie class structure surrounding positional data that takes up far more memory than the container map entries then the addition to a container is probably a quarter of the bloat compared to leaving it on the ground. So while it does help it's not entirely negligible. If it were an order of magnitude lower then I'd scoff at it, but the possible values for reference ids suggests they aren't using 16 bit short values or anything tiny like that.