Engine size matters along with gearing. Mom's 3800lb Buick Ultra w/SC'd 3.8v6 will get up to 30+mpg on the hwy due to gearing. Elantra seems to be geared similarly, but it lacks the power to do anything but flat land. While the Buick only has to spit out a bit more gas for the oomph to get over that hill, the Elantra has to chug a bit.
And thats def not a lot of work, thats actually good driving, and barely tapping the hypermiling potential. A further step you could do on downhill sections is to pop it into neutral, some take it even farther by turning the car off but not a fan of that. I only turn off at a recently turned red light.
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