Saving large custom MATLAB objects and MATLAB save versions

When saving large custom objects in the following scenarios I encounter MATLAB running out memory and fully crashing. No red text is displayed but matlab beeps perioudically and my RAM usage spikes in a sawtooth pattern (gradually increasing till a limit then dropping back to a start point and repeating).

Object A: Large custom object containing various data types including structures and other smaller objects

Object B: The same object as A but created using a newer verison of tooling which leads to less data being stored.

Using different save versions in matlab I am able to gather this information.

% Save Version	    v7	    v7.3
%               A 	150Mb	3.1Gb
%               B 	Errors	1Gb

I do not understand why I cannot save object B in version 7 since it would seem to be a third of the size of object A. Obviously there are many benefits to saving in v7 which is why I would really like to understand what is causing this issue.

[Matt J]

No way for us to know, since we don't know what either A or B contains or how they are built.

[George Hackney]

I thought that might be the case, what would you need to have an idea of what may be the cause. I cannot provide the code or objects as they are stored on an offline air gapped system.