Deep service: Numbers maintained through a smaller number of people who serve for a medium to long term. Say 6 years to retirement for average service length. Small percentage of eligible population actually serves.
Wide service: Numbers maintained through a large number of people who serve mostly for a short term, with a core of "professionals" who stay for a career. Sweden is like this. Our forces in WWII-Korea-VN were like this. In a lot of countries, everyone or most everyone eligible serves, either through mandatory conscription, patriotism, or a fairly kick ass benefit plan. I forget if it was Sweden or Finland, but I remember talking to some sailors on BALTOPS and they were on a "do two years of voluntary national service, get college for free" plan. Think GI Bill on steroids.