The Bosendorfer Library (400MB+) and the Chick Corea Mark V (400MB+) are examples of single instrument Libraries. Each come with 16 Performances. Whether you wish to load one Performance or all sixteen Performances, the amount of Waveform memory used is the same.

In other words, all the Performances in the Bosendorfer Library refer to the same Bosendorfer Imperial 290 Acoustic piano Waveform data.
All the Performances in the Chick Corea Mark V Library refer to the same Rhodes Mark V Electric piano Waveform data.

You save no memory by selecting to load less Performances.

What you can do:
You want to ensure that the Waveforms you have elected to install are essential. When you acquire a new Library you should take your time and decide, each-by-each, what data you absolutely wish to make a *permanent* part of your instrument.

Among the factory data, Yamaha technicians/musicians and programmers selected the 6,347 Waveforms that form the instruments permanent resource — some 5.67GB of data... think of your 1GB Library area, as your opportunity to add to the Wave ROM resource.

You want to avoid adding any data, if you are not going to use it. The biggest mistake users make when assembling their own custom Wave data is they are in too big of a hurry to make the decisions and install the data. Take your time. First, make the decision to add each sound carefully. You will discover some Libraries have many different instrument sounds, making it more difficult to know what Waveforms you need to keep and which ones you can delete. (This is accomplished by deciding on which Performances appeal to you).

As you know from above, some Libraries are a very, very detailed reconstruction of one particular instrument sound (an acoustic or electronic piano) ... others may have scores of samples from dozens of instruments.

The assembly of your own custom libraries, takes time and some effort on your part.

Ok, so, the Bosendorfer and the Chick's V Libraries have only one Waveform data ( valid for all the 16 performances included) = 400MB+
if I understand correctly even if I decide to install the entire library and then delete 14 performances out of 16 to keep only 2 performances, the amount of space occupied in the internal memory of the MODX will not change and will be the same, right?

Actually I don't have any Custom wave data I only have two Yamaha libraries inside the MODX : the Montage Expanded and the KAPro CS80
so I have no more free space to add the Chick's V or the Bosendorfer libraries unfortunately....it's really a shame because inside these two libraries there are some sounds that I love and that I'd like to have inside my MODX as a permanent part of it

said that, to free up more Wave ROM resource is there a way to delete some single sample Waveforms from the currently installed libraries? I mean some single waveforms from the KaPRO CS 80 and the Montage Expanded...I'm sure that you'll mention John Melas but it's not a cheap software

Go through the “Montage Expanded” and the “KA_ProCS80” write down the Performances that you deem are keepers... use the “Library Import’ feature to move those to an initialized User Bank (or to join other User Bank Performances) — these will be saved as your new streamlined Library.

You can then DELETE the Library Waveforms. When you install your new streamlined Library, it will not contain any of the unused Waveforms.

" ....You can then DELETE the Library Waveforms.." do you mean : Contents > Data Utility > Montage_Expanded > Delete?
"...When you install your new streamlined Library, it will not contain any of the unused Waveforms.." do you mean the new one ? for example the Bosendorfer?

No, I mean once you select the Performances (from the MONTAGE EXPANDED and KAPRO_CS80) that you deem essential... use “Library Import” to move them all to the User Bank/User Waveform Memory — you’ll then save that User Bank as a new (streamlined) Library File. At that point you can Delete the two installed Libraries. This will reduce your installed Libraries to zero.

When you install the newly create Library File it will be smaller because it will contain only the Waveforms for the Performances you selected.

Now if you’ve reduced it enough, the Bosendorfer Imperial (400MB) might fit.

Done, now it fits...

Excellent!

Is there a way to edit a library's waveform: deleting some keybanks and stretching the key/velocity ranges of the remaining ones to compensate?

This way we could shrink the size of many libraries and still have waveforms (and patches based on them) that are still "good enough" for most ears!!

By the way, most of the time, library vendors just inflate the samples weight to look better (after all a 4+ GB piano MUST sound better than a 50 MB ones!). This is grossly wrong. And the (tiny, if compared to VSTs and SSDs capacity that nowadays sell for peanuts) sample rom shipped with any Yamaha synth is a testament of this.

The whole MONTAGE/MODX ROM + plus user area fits into a 4 GB flash rom chip. Since the Montage user area is 1.7 GB (probably artificially limited to 1GB on Modx just for marketing reasons), then all the 6000+ waveforms are squeezed into a mere 2.3 GB of (compressed) storage (the 5.7 GB quoted on spec is just a marketing trick to look better, because this number is before any compression/downsampling/bit reduction being applied).

And why we are not allowed to compress samples as Yamaha does for the sample rom? I mean, you load all your wavs, assemble the waveform and then apply the command "compress". It does not seem rocket science to me!