Weasel
👄 I'd intercept me
PDO's main benefit (also IMO) over MySQLi is multiple driver support. MySQLi is (admittedly insufficiently) faster than PDO, and supports a procedural API as well as OOP (PDO only supports OOP). PDO seems overkill for this "cms", its not really a CMS but still.
Benchmarks show that MySQLi outperforms PDO by 2.5% on unprepared, and 6.5% on prepared. That and the fact that PDO claims to only support "most" statements and MySQLi supports all I just think MySQLi is a better choice.
Also... "only 30mb" ?? 30mb isn't exactly tiny for a "simple cms".
There's quite a few more differences between PDO and MySQLi than just speed, which especially in this case would make literally no difference at all.
In the end, as with most cases, it comes down to what you need and what you prefer. For example, a reason I would personally never switch to MySQLi (unless really needed for whatever reason) is due to their use of snake_case in their function names.
But don't come along here trying to be smart telling people to stop using something for bullshit reasons.