Optimising memory used by Apache httpd process

I am comparing our 2 VPS - one with Centos 6 and the other with Centos 7. I am trying to figure out how to reduce httpd memory consumption on Centos 7. On Centos 6, the memory usage by httpd is something like this: ps aux | grep 'httpd' | grep -v grep | awk '{print $6/1024 " MB\t\t"$11;}' | sort -n 8.17891 MB /usr/sbin/httpd 15.789 MB /usr/sbin/httpd 26.5048 MB /usr/sbin/httpd 31.3944 MB /usr/sbin/httpd 32.043 MB /usr/sbin/httpd 32.477 MB /usr/sbin/httpd 33.591 MB /usr/sbin/httpd 33.602 MB /usr/sbin/httpd
However on Centos 7, there is a big spike in memory usage: ps aux | grep 'httpd' | grep -v grep | awk '{print $6/1024 " MB\t\t"$11;}' | sort -n 6.04591 MB /usr/sbin/httpd 8.05974 MB /usr/sbin/httpd 14.9014 MB /usr/sbin/httpd 122.106 MB /usr/sbin/httpd 124.843 MB /usr/sbin/httpd 126.05 MB /usr/sbin/httpd 127.709 MB /usr/sbin/httpd 128.02 MB /usr/sbin/httpd 128.98 MB /usr/sbin/httpd
The directives on `httpd.conf`files on both the systems are pretty much identical - the prefork directives are same on both systems. There are around equal number of sites hosted on each VPS. Perhaps only a couple of more sites hosted on Centos 7. However the sites with higher traffic are hosted on Centos 6. The only significant difference I could find between the 2 systems, in terms of httpd, is that Centos 7 has more httpd modules installed than on Centos 6. Would that be the reason for the higher memory usage? Any other suggestions would be greatly appreciated. Thanks