I can understand why some players would judge Big 4 entry by PVP standards, but for me (mainly playing PVE) that's not the main criteria, for me it comes down to:
It is big?
Is is desirable? Do I want it?
Can I survive the majority of PVE threats in it?
Everyone will have their own ideas of what defines a Big 3 (or 4 or 5 or 6 etc). For me the T10 can and has joined the club, I'm sure others will disagree and that's fine.
The buffed T9 misses out IMO, an extra 256 tons of cargo is nice, but won't be much good vs 2 angry FASs. If anything those extra tons will slow you down, potentially making your escape harder - more so if mass locked (which can be the case vs FAS, bizarrely).