Catalonia comprises about 16% of the population of Spain, whereas Scotland comprises about 8% of the United Kingdom's population.
Catalonia is more economically and demographically important, but Scotland is far more strategically and militarily important, makes up a bigger part of the country's territory and is more important for national identity. Spain would still be Spain without Catalonia while Britain would be reduced to the old Kingdom of England (plus six Irish counties which would be an even more pointless appendix to the state than they are now). I suppose you could still call it the United Kingdom (of England, Wales and Northern Ireland..) and keep the Saltire in the Union Jack, but somehow that would be a bit phony. Losing Scotland would make the UK a Greater England whereas losing Catalonia wouldn't make Spain a Greater Castille.
I don't think Spain would be still Spain without Catalonia. I know that most of the people abroad tend to identify Spain to Castile, in a similar way non-British people often identify the UK to England. Castile was the most powerful of the medieval kingdoms that made up Spain, but Catalonia was the most important part of what once was the Crown of Aragon. Spain is not a compact nation state, but rather it's a state with a plurality of nations and a great deal of regional particularism. Spain without Catalonia would be little more than a rump state centered in Madrid, I have little doubt about it
Catalonia is only 6.3% of Spain's territory, it seems bizarre to claim the remaining 93.7% of the country would be a rump state, a remnant of a once much larger state. This isn't post-Trianon Hungary we're talking about.
Both Britain and Spain have multilayered identities, but while Spain includes a lot of distinct cultures it is stil not a state that's defined as a union of separate countries the way the United Kingdom is, a view that's reflected in its national flag and other symbols. Losing Scotland (which would likely also lead to the exit of Northern Ireland) would require a redefinition of what the nation is which just doesn't seem like a realistic prospect in a post-Catalan secession Spain.
The Basques and Catalans may view the Basque Country and Catalonia as countries, but constitutionally they're mere regions. The UK would lose a whole geo-strategic space without Scotland (the North Atlantic) while Spain would still be a Mediterranean country without Catalonia.
btw I don't think foreigners view Spain as Castille, which isn't that well known outside of Latin America. Spain is mostly seen as Madrid, a lot of empty space in the interior and a ring of coastal regions with different cultures and sometimes local languages in a half circle around it plus some tourist islands. It then varies how many of those regions people know about, with Andalusia and "the Barcelona region"/Catalonia being the best known. If anything foreigners will tend to underestimate how many people actually live in central/interior Spain outside of the Madrid metro area.
