What aircraft are these, and why are these engine sizes so different?
(Left), (Right)

On the left is the Pratt &amp; Whitney J75 (JT4A) on a DC-8, with ram air inlets1.

On the right is an RR RB211, possibly on an L-1011. The EPR probe (I think) can be seen protruding from the inner lip at the 12 o'clock position, plus the snubber2 holding the blades together.



The former is a turbojet, that's why it's narrow. The latter is a high-bypass turbofan, that's why the big fan.

We can't directly compare both engines. The RB211 is 14 years younger, and much more powerful, suited for big planes unlike the narrow-body DC-8. The RB211 wouldn't fit under the wings of the DC-8. Different planes, roles, and engine requirements.

(Source) DC-8 with the high-bypass CFM56.

The DC-8 was later retrofitted with high-bypass because it's better for fuel economy and noise—the CFM56 to be precise—without the DC-8 getting the retrofit, the CFM56 would not have become as ubiquitous as it is now. The project needed civilian customers to survive3.

First run years: J57 (1955), RB211 (1969), CFM56 (1974).

L-1011 (top) and DC-8 (bottom). Eyeballed the scale.



1 Accessories on the Pratt and Whitney engines are mounted beneath the high-pressure compressor casing. Two drum-type oil coolers are provided, each fed with air from individual ram intakes in the lower part of the nacelle.— Flight
2 A snubber (or clapper) is a damper used to prevent blade flutter on narrow-chord fan blades, at the cost of reduced efficiency. Hollow wide-chord blades are more stable and do not need snubbers.
3 The CFM56 featured article on Wikipedia is a must read.