It is unlikely that the largest supermassive black hole in the universe has already been discovered, there may be much larger ones.
Are there, or could there be, supermassive black holes large enough to swallow whole galaxies as opposed to the occasional star?
Are there other factors beyond size that would also affect a SMBH's ability to swallow it's own galaxy?
This question might be related to Is there a maximum size for a black hole?
Typical SMBHs do not contribute much to the overall mass of a galaxy, much less than the sun is of the total mass of our solar system. And as we can see, even such mass centered quite nearby does not swallow all objects in the solar system very easily.
While there is no theoretical maximum, in practice - based on simulations - maximum mass for a black hole seems to be around 50 billion solar masses†, which will give a radius of 0.01 light years. This is very small compared to the size of the galaxy, so it will not affect the situation much. Thus the main effect of huge black hole vs galaxy would be via plain old gravitation.
The main impediment for a black hole to swallow anything is angular momentum. Any object a black hole is going to consume must shed its angular momentum in order to get closer to event horizon. This is not very easy thing to do, and the main reason black holes are surrounded by discs of extremely hot gas - friction will turn angular momentum into heat, allowing matter to descend towards black hole.
†practical maximum of 50 billion suns is due to fact that accretion disc becomes unstable due to new stars forming from it. See for example Andrew King's 2015 paper How Big Can a Black Hole Grow?
Abstract:
I show that there is a physical limit to the mass of a black hole, above which it cannot grow through luminous accretion of gas, and so cannot appear as a quasar or active galactic nucleus. The limit is $M_{max} ≃ 5 \times 10^{10}$ M⊙ for typical parameters, but can reach $M_{max} ≃ 2.7 \times 10^{11}$ M⊙ in extreme cases (e.g. maximal prograde spin). The largest black hole masses so far found are close to but below the limit. The Eddington luminosity $≃ 6.5 \times 10^{48} \text{erg s}^{-1}$ corresponding to $M_{max}$ is remarkably close to the largest AGN bolometric luminosity so far observed. The mass and luminosity limits both rely on a reasonable but currently untestable hypothesis about AGN disc formation, so future observations of extreme SMBH masses can therefore probe fundamental disc physics. Black holes can in principle grow their masses above $M_{max}$ by non–luminous means such as mergers with other holes, but cannot become luminous accretors again. They might nevertheless be detectable in other ways, for example through gravitational lensing. I show further that black holes with masses ∼ $M_{max}$ can probably grow above the values specified by the black–hole – host–galaxy scaling relations, in agreement with observation.
