Hi
Apologies to hear that, but in the most of cases the recent IOS releases have other MB size because they include new features to be aligned to new technologies or protocols and they include improvements/fixes that previous images don't have that is the reason to increase the size. Now you can acquire a compact flash with more capacity usually, it is low cost. My suggestion is contact an authorized Cisco Partner to know the costs depending of the Compact Flash size.
The following link can be useful to meet the capacity supported or hardware requirements:
https://www.cisco.com/c/en/us/td/docs/routers/access/800/hardware/notes/800upgrd.html
Also I recommend verify the hardware required for a new Image before to upgrade.
Hope it is useful
:-)
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