The size of the codec is so unimportant as to be essentially irrelevant within reason outside of a few extremely niche cases. We care about video file size, we care about subjective quality, we care about power efficiency in compression and decompression, in many applications we care about latency; on all these metrics, modern codecs are stunningly well-optimised. Modern codecs are extremely complex, but that complexity is absolutely necessary for them to perform well on the dimensions that actually matter to users.

Nobody uses Theora, because it's a bad codec. It was worse than H.264 back in 2009 and it hasn't been updated since. Removing support for dead formats is generally a very good idea, particularly in a web browser, because it reduces the attack surface; we have recently seen a number of major vulnerabilities caused by archaic, neglected file formats and codecs that provided almost no value to users.