Transistors at high frequency
More details on where you read this would provide context to narrow down answers but here are some general points that may help.
&quot;smaller&quot; could mean in terms of physical size or in terms of power capability.
Physical size - as frequency increases, parasitics (such as series inductance) need to be smaller. Transistors in physically larger packages tend to have higher parasitics which can degrade the ability to match impedances. Difficulty can mean hard to match broadband or ending up being very sensitive to slight changes in transistor characteristics (e.g. over temperature) or component values.. At very high frequencies the package interconnect can electrically be considered a transmission line or Integral matching component.
In terms of power, as frequency increases, the maximum practical power that a single transistor can provide is smaller. Primarily it's because a power transistor consists of many small transistors connected together in parallel. The input signal splits into each mini transistor, is amplified, then is combined back together. This needs to happen such that the signals combine in phase. As frequency increases, the feed networks in and out of the mini transistors becomes more complex and more lossy, rendering the transistor less efficient.
In terms of overall circuit size, it's hard to generalize too much. However, components like capacitors, inductors and transmission lines tend to scale in value and in size inversely with frequency. So, an &quot;equivalent&quot; amplifier at 10GHz will be smaller than at 1GHz or 100MHz.