Filling a bundle with larger than intended stacks extends the progress bar past its limit.

Giving a player a bundle containing more items than it should be able to fit, for example 127 glass via the command "/give @s minecraft:bundle{Items:[

{id:"glass",Count:127}

]}", causes the blue progress bar to fill up beyond its normal limit.

Setting the item to a 'larger' item (e.g. tools or boats) fills the progress bar even further.