Bacteria, individually, are incredibly tiny, (about 1 picogram - one trillionth of a gram) (or 600 billion daltons in molecule talk).
Tiny, even when compared to a typical human red blood cell (which are about 30 times heavier).
Just as penicillin molecules are tiny compared to average large biological molecules like human blood hemoglobin
(it would take about 200 penicillin molecules to weigh as much as one hemoglobin molecule.)
(And human titin molecules can weigh almost 4 million daltons !)
But perhaps the most useful index is to compare the size of the tiny penicillin (300 daltons) or sulfa molecule (250 daltons) to the tiny bacteria organism it is trying to kill.
At least a billion times smaller.
That makes them highly diffusible, in theory.
Because the danger is always that once inside a living body, they will be more attracted to other molecules than to their designated target molecule. A very common situation that remains one of the great uncertainties of drug discovery.
Still they are pretty diffusible, all in all - approaching the real tiny and hence very highly diffusible molecules like water (18 daltons).
And to chemists, trying in the interwar years to determine the molecular structure of big biological molecules like hemoglobin, the tiny sized penicillin seemed as easy to synthesize as the similarly sized synthetic sulfa drugs.
(Or their most recent biological synthesis success - man-made Vitamin C, weighing in at about 200 daltons.)
But quinine also weighs about the same as penicillin and the best chemists throughout history had failed to synthesize it commercially.
Size, it turned out was not the best indicator of the complexity of making penicillin molecules synthetically....