Sort(a:a) changes my 120 spreadsheet size to 1M+ filled with zeroes

Essentially, SORT is performing an EXPAND (albeit, involuntarily). It's interesting because Excel can perform this operation quickly. According to this guide, SORT uses approximate matching, so it makes sense.
https://docs.microsoft.com/en-us/office/vba/excel/concepts/excel-performance/excel-tips-for-optimizing-performance-obstructions#speed-up-lookups