Let's take a functional $F[\phi]$ as defined in this answer
$$ F[\phi] = \int d^4x \, \phi\, \partial^2 \phi $$
whose dimensions are, if the coordinates have dimensions of a length, as it's customary, are $[F]\, = [\phi^{2}]\,[L^{-2}] [L^{4}]$
It's functional derivative is $$ \frac{\delta F[\phi]}{\delta \phi} = 2\, \partial^2 \phi $$
This means that the dimensions of the $\displaystyle{\frac{\delta F[\phi]}{\delta \phi}}$ are $[\phi]\, [L^{-2}]$
which is equivalent to say that, assuming $[\delta\phi] = [\phi]$, $$ [\delta F] = [\phi^2]\,[L^{-2}] = [F]\, [L^{-4}] $$
Is this correct that $F$ and $\delta F$ have different dimensions? I find it a bit weird and I don't actually don't get why this happens.