Just jumping in here - I have seen this happen before with report builder.
We have a report that is 400kb and VERY detailed (1 evar that has 20 variables x 7 days x 3 devices x 10 marketing channels) - which then has maybe 10 metrics. This is normally 400kb(ish) however when we ran it on an other report suite it jumps to 8 meg. The number of rows/columns/design stay the same, so the file size should be very similar but it appears not.
Obviously at 8 meg we can't run schedule it so it has to be manually run by staff and emailed out.
Are you absolutely 100% positively sure that's literally the only thing in your workbook? I have workbooks with multiple requests and more lines of data, and they're only 20KB. I'm pretty sure I can fit multiple requests with 50K rows and still not even hit 1MB.