File Size
File size of a 3D are mostly from the polygon count and vertex count. This is how many polygons and vertexes are in the model. A realistic third person game needs higher polygon counts than other games for the character since they will always be visible because of the third person view, and need to look detailed as it is a realistic art style.
A lower polygon count would be used on a first person game as the characters won't always be visible, so they don't need the extra detail.
Polygons and Triangles
When creating a model artists generally want to work with easy to use polygons such as quads(4 sided) as they are easy to use with edge loops. An artist would want to preserve the easy to use polygons for as long as possible to make creating a model as quick and easy as they can.
The problem with this is that game engines are better at rendering triangles. The models polygon count is an inaccurate representation of how many polygons there will actually be, since when the model is placed into the game it will be broken down into triangles and therefore more polygons.
Triangles and Vertex Counts
The vertex count is how many vertices(corners) are on a model. The vertex count is more important than how many triangles are on the model in terms of memory and performance, however the two counts can be similar if all of the triangles are connected properly to each other. 1 triangle has 3 vertices, 2 connected triangles have 4 vertices, 3 connected triangles have 5 vertices and so on.
Seams in the UV maps, changes to shading groups, and material changes between each triangle are treated as physical breaks in the model's surface when it is rendered in the game. When these physical breaks occur the vertices have to be duplicated so that the model can be sent in render-able chunks to the GPU.
Using too many smoothing groups, too many materials, and over-splitting of UV maps are all causes to having too high of a vertex count. Having too high of a vertex count can slow down performance when the model transforms, and also increases how much memory is used up for the mesh.
Rendering Time
Real Time Rendering - This is for interactive media like games and simulations. All of the calculations are done quickly so the image can be displayed in a fraction of a second. Each image is called a frame, which display movement when chained together quickly. It is best to aim for at least 30 frames per second, but 60 or higher is best. The target is to have as high of a frame rate as possible to show as much information as quickly as possible, keeping the animation smooth.
Rendering software can simulate real life by adding visual effects like motion blur, depth of field and lens flare. Real time rendering is done using the GPU.
Non Real Time Rendering - This is for animations which aren't interactive, an example would be a film or TV show. Because non real time isn't trying to display the image quickly it can sacrifice time for better image quality, producing much more realistic or detailed animations. Each frame could take anywhere between a few seconds to a few days depending on the complexity of the scene. Once all of the frames have been rendered they are displayed quickly and, just like with real time rendering, the more frames displayed per second the smoother the animation is.
When photo realism is needed, ray tracing or radiosity can be used. Particle effects can be made to simulate weather or environmental effects like rain, fire, smoke or explosions. Volumetric sampling can be used to create atmospheric effects like fog. Caustics simulate light focusing by uneven light refracting surfaces. Subsurface scattering simulates the effect of light reflecting inside of a solid object.
Rendering times can be extremely long in some cases, and because of this a 'render farm' can be used to speed up the process by using lots of hardware to render images simultaneously rather than one at a time. However because of the increase in computer technology and lower prices for more powerful hardware rendering of complex scenes can be done on one computer system, and can even be done outside of a professional environment.