If the strings are too thick, they may not fit through the tuner posts or they may be too difficult to turn.
In either case, the strings will not be properly tensioned and the guitar will not sound correct.
Additionally, using strings that are not the correct size for your guitar can damage the instrument, even if the strings do fit through the tuner posts. Strongs strings can put excess strain on the neck of the guitar, which can lead to warping or even breakage. Similarly, strings that are too thin can cause the guitar's intonation to be off, making it sound out of tune, and can potentially damage the frets.
Therefore, it is important to use the correct strings for your guitar. If you are unsure of what strings to use, consult with a qualified guitar technician or refer to the manufacturer's recommendations for your specific guitar model.