You might "think" you can, the retailer may tell you the gain of their antennas is much higher, but we'll explain why this is not true.
Firstly we'll focus on antennas in the 700 - 900 Mhz range. These are the frequencies used in longer range mobile communications and covers Telstra, Optus, Vodafone and all of their MVNO's (Mobile Virtual Network Operators) like Boost, Amaysim, Aldi Mobile etc. This is what you'll find in use outside of built up areas.
We sell a yagi with14 dBi gain across 698-890 MHz - designed for Telstra 4gX and NextG. The antenna has been designed to work optimally on these frequencies but never the less is 1300 mm long. If it was possible to make it shorter the designers would have - easier to manufacture, cheaper, easier to install. But it is 1300 mm because physics demands an antenna on these frequencies delivering this gain to be this long.
This is where other retailers and some manufacturers make unscrupulous claims. A quick online search finds one retailer offering an 18dBi Yagi ca. 1200mm long covering NextG, Optus etc. for $89. How do we know this is b***s**t ????
Because if we take an antenna that is 14dBi, and accept the designed length is ca. 1300mm (can be slightly less if optimised for one frequency) a quick calculation to get even just 17dBi means we have to double the length of the yagi for every 3dBi extra gain. So to get even near 18dBi the antenna would have to be more than 2.5 metres long!
And if your, curios about why there are no "tricks" to make a high gain antenna short - it's due to the wavelength of the received radio wave, which in turn is determined by the speed of light - so unless they can prove Einstein wrong these short but high gain antennas are breaching the laws of physics!!