There are several ways to answer this, but perhaps a starting point is to ask why we think they're the same. If I ask you the size of your swimming pool you're more like to give me an answer in feet or metres that to reply with the number of person days it took to build the pool. (Building swimming pools is an xProcess application by the way - but that's another story!) When we are discussing the planning of a project the first thing to consider is the size (in whatever units are appropriate) of the deliverables of that project. Then we need a function (based on previous experience) to map from size to the estimate of the effort and consumables required to provide those deliverables.
Because the size of the things we want to deliver are measured in many different units, all of which we want to map eventually to an estimate of effort, it is common for projects to estimate directly in effort units such as person-days, person-hours or person-years. However if we do this, we lose a very important measure of the effectiveness of the project. Questions of team velocity or productivity become somewhat meaningless (except as a correction factor on original estimates of effort, compared to actual effort expended).
So if we can agree size is an important metric for projects, the next question is what units should it be estimated in, and can we compare the estimates with actuals at the end of the project. Software projects in particular are notoriously difficult to find an appropriate measure for size. Lines of source code is commonly used (because they are easy to count) but they don't readily map either to effective functionality delivered nor to effort required to develop them. Function points is another option and more recently "story points" (for an XP user story) or simply "points" have gained popularity where these are arbitrary units that are comparable only within the single project where they are being used. Another useful measure is "ideal-days" - in other words the amount that can be produced by a typical team member in a completely uninterrupted day. We rarely get such days (if ever) so it will take most of us more than a day to implement an ideal-day's worth of size. Nevertheless it is an understandable and reasonably verifiable measure of size.
In xProcess the units of size are common to the whole Data Source (they can be set in Data Source Preferences) and the value is used in several important places. When creating tasks the size value is often used in process patterns to set initial values for three-point estimates. The size units are used in burndown charts and earned-value plots to show progress of a project towards creating its deliverables. And size is also used to calculate productivity measures such as team velocity and software productivity.