Some races will go, but races will be more detailed. Some classes will go but there will be more options. Let's say that cancels out. The combat section in the 3.5 PHB is surprisingly short - 28 pages. I don't think much can be cut there.
The behemoth of the PHB is the magic section. 135 pages in total, 108 of which are separate spell descriptions. I figure you could easily trim 50 pages, maybe more. So many of the spells are unnecessary, and have their own little rules.
Also fewer classes means fewer spells. Maybe no bard, maybe no druid. Rangers probably don't cast spells any more.