Banana Plugs too narrow

Were they cryogenically treated?  Extreme cold always makes them shrink.

A good, well designed banana plug should fit snugly.  I'd return these if they are as you described.  Hopefully just a bad set.  Yes, you can widen some bananas with a split pin, but this tends to fatigue the metal.  You'd soon be back where you started.  I'd get my money back and go for a better set.  

C'mon guys, everyone knows it's not the size of the banana, it's how you use it that counts. 
Generally speaking insert as deeply as possible and then tighten your nuts 

Sophomoric humor.  I guess people who understand audio engineering find Caddyshack to be the epitome of humor.  It must be a generational thing. 

I've been away from the 'Gon for about a year and a half.
You guys are still as witty and hilarious as you were when I left.
LOL!
It's good to be back.

Tricky old audiophile? No, that's not it.

Don't worry. I'll keep looking!

It took several videos, but the interweb really does have all the answers. Bananas have split sides. Stick a screwdriver in there, twist it sideways, widen the slit. This will increase your girth. But you will lose a little length. Your banana will be thicker, but shorter. It just won’t go as deep, even when you are all the way in. Sorry. But in spite of everything you may have heard there is no way to make your banana thicker and longer. If you want to see how this is done do a video search for don’t break me. Or was it speakers love big bananas?

any chance you can change out the connector for locking banana plugs?

Are they locking bananas? If so, you need to tighten the collar after you slip them in. This expands the post for a tight fit.

When I’m plugging into my speaker, I prefer the D’appolito position.
Too much? ok.

Wouldn't risk going bare, even if you are pretty sure where that speaker has been, it could have had a spade on the side. And.. wait, is this a 3-way we're talking about? Or ... five???

Out of curiosity, what speaker cable are you using?  If I were truly smitten with the cable I'd inquire into sending them back to the manufacturer and have them replace the bananas with spade connectors.  A few years ago I did this with a pair of Kimber cables.  I think it cost me about $60 shipped working with Kimber. 

Here is a suggestion on what you can do if you have a leftover banana. :-)
https://news.sky.com/story/banana-duct-taped-to-wall-art-installation-sells-for-120k-in-miami-118792...

Simply slipping in and out of the hole makes for a very unsatisfying listening experience.
Exactly how many owners have passed them around?
Anyway, yes, you can snip off the bananas and use bare wire.
But don’t just jam them in your speakers hole.
Take your time. Slowly twist the binding posts, gently at first, then faster and with more force as they start to loosen up. Then when you think they’ve reached the point where they are on the edge of coming off, slip your bare wire behind them, gently twisting them the opposite way until they are so tight you can’t do it anymore. Now, turn on your stereo. Or smoke a cigarette....

Unscrew the binding post. If in the post there is a hole, put the bare end of the speaker cable into the hole and tighten the nut. If it doesn't have a hole, take the bare wire and wrap it around the post and tighten the nut. Make sure they are tight.

BTW, the humor concerning engorgement is acknowledged, but really, not very witty.  Sorry.  My assumption is males who are fixated on their appendage, must be very insecure about said organ. 

Can I simply remove the plugs and use bare wire? 
Can I insert the plugs in the binding posts where the bare wire would go? 
The wire and plugs looked normal to me:
Tuneful cables Belden 9497, Cotton jacket, silver beryllium BFA.
The owner/manufacturer was very nice and spent an inordinate amount of time with me, but his wire simple had no connection.  It just slipped in and out of the hole in the binding post.
Thanks

If the have the Z ends you could try widening then with a pair of needle nose pliers.

There used to be ads for this in the back of every magazine. Nowadays though I think you have to do a search for banana enlargement.

Did you get Furutechs?  That happened to me.
Switch to WBTs.

You need a bigger banana.