Are they really that big? They get good attendances, but that only really demonstrates their popularity in Scotland - match day income represents a miniscule proportion of PL clubs income.
Do you see kids wearing Rangers/Celtic shirts? Not really, other than in Scotland.
Do they have large a large appeal on social media? Not really, 850k followers on Twitter: nothing compared to the top 6 PL clubs; Arsenal, Chelsea, Man Utd and Liverpool have over 20m. Even the blades have 500k.
Do they have any appeal in overseas markets; China, North America? I'd guess no, the value of the SPL TV rights certainly suggests not.
If you measure a clubs size by the old fashioned metrics, yes, they're undoubtedly big clubs, and had they been playing in England for the last 3 decades, I'm sure one or both would be comparable to the top 6 in England.
By modern metrics, the ones that count to investors, they're small fry, unfortunately for them. The idea they'd be able to catch up with the better teams in England is frankly ludicrous if you ask me.