bigger balls i'll say
Details, articulation, and features are often different. I don't know if you're focused primarily on the letters "HG" or if you're thinking about "MG" as well. In recent years, Bandai has stopped labelling their 1/100 TV series based kits as "HG". Now, the labelling is
1/144 TV series HG (and also HGUC, which is simply 1/144 HG for all kits falling within the fictional UC century in the Gundam Universe).
1/100 TV series
1/100 MG - Master Grade - totally different ballgame - these all have an internal frame, many more parts and more details/features. These are usually styled differently too and have a sharper, more contemporary look (exceptions apply - especially for recently released kits) - also cost a lot more too.
The old labelling, and we're going back to almost 10 years ago - included HG on 1/100 kits (for Gundam G, X and Wing). You could pretty much treat those as larger versions of the 1/144 kits with more panel line details. Articulation is pretty much the same as their smaller counterpart but they usually do have a couple more features that were not available in the 1/144 version.
Sounds like you're buying this as a present? My suggestion is this: pick the kit based on the particular model you want, then get the one that suits your size and budget.