Fatter looking modern engines
The posts so far have given the basic reason for the bigger diameter and fatter shape (a larger fan in the high by-pass engines).
On the attached 737 pics you see on the 1968 Britannia A/W shot with the original JT8D pod Joe Sutter decided to put under the wing of the 737 by moving a cardboard cut out engine around on a drawing till it looked right! But it created drag and Boeing had to supply the longer pod in your Continental photo to all operators (at Boeing’s expense, I suspect)
The second photo shows that later pod on a SudanAir gravel runway kit equipped 737…note the airjets to counteract the ‘hoover’ effect and the nosewheel gravel deflector
Shoe horning the fatter CFM engine into the 737-300 (now ‘Classic) was achieved by mounting the whole engine further forward so the big fan could sit higher . I believe the 737NG has a slightly longer undercarriage making a rounder pod possible
If you go back to the 40s the first British engines were fatter looking than later engines because they had centrifugal compressors which had ducting on the circumference…. compare the Meteor engines with the axials on the Me262
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