If you just summon an item, which is a bundle containing more than 64 items in total, the bundle will show a big progress bar.

The command I used for that bundle is: 

/summon minecraft:item ~ ~ ~ {Item:{id:"minecraft:bundle",Count:1b,tag:{Items:[{id:"minecraft:diamond",Count:64b},{id:"minecraft:dragon_egg",Count:64b}]}}}

But if I change it to have even more items, the progress bar will fill up the entire screen