Why did Lithuania get so little land after it's independence in 1918, considering it's great territory as Grand duchy of Lithuania?

Short version: Because Republic of Lithuania is entirely different entity build on entirely different basis.

Longer version: The roots of the modern Republic of Lithuania lies in the National Awakening movement, analogous to the other National Revival movements across Europe and was primarily concerned with needs and interests of ethnic Lithuanians. At the early stages Lithuanian National Awakening was concentrated with cultural and educational needs, as it was with all National Revival's and in Lithuanian even more so due to press ban. In fact the beginning of National Awakening in Lithuania was book-smuggling and underground school network launched by bishop Valančius.

With rapid Polonization of Eastern Lithuania being one of the main threats to the emerging ethnic nation and general hostility movement suffered from Polish (there are numerous and complex reasons of this I will not get in to here- it would require a lengthy and unnecessary for your question detour), Lithuanians at the time had a rather skewed consensus on history: everything after Vytautas the Great was simply bad, with Poland being first and foremost source of all evil and Russia holding a distant second. The good part of history- source of pride and inspiration- was Vytautas era and before. With latter period being willingly ignored, this era was to remote and disconnected to be actualized as political goal (This looks like very simplistic view, but first Lithuanian historian defended his thesis in 1905, so yeah... there's that).

Such perception of history with Polonization and Russification being foremost problems formulated a rather different goals than to chase after medieval borders.

Lithuanian National Movement first expression as political force was during First Russian Revoliution (few political organizations already existed but any impact they had was rather marginal). The resolution of the resolution Great Seimas of Vilnius (December 21-22, 1905) defined territorial claims of the Lithuanians as ethnographic Lithuania as a core with whose borderlands that are attracted to it due to economic, cultural, ethnic or other reasons and inhabitants [of these borderlands] agrees to it.

While demand of autonomy in 1905 evolved in to declaration of independence in 1918, territorial claims never extended outside ethnic Lithuania. There is entire library-worth of articles, books and pamphlets dedicated to discuss exact borders of this ethnographic Lithuania, however there was simply no-one arguing for the "Greater Lithuania".

As for the title itself: Seimas of 1905 envisioned Lithuanian autonomy as the Grand Duchy within Russian Empire led by democratic parliament elected in universal suffrage (without making any distinction of gender, nationality, faith), however in that time and place monarchy itself was associated with autocracy and monarchic titles as "emperor", "king" or "grand duke" were seen as a contradictory to democracy.

And, as democracy was seen just as core tenant as ethnographic Lithuania, the new polity was a republic with intention to encompass all ethnic Lithuanian lands. Anything else was simply outs of the question.

N.B. This, in general, is not exclusive to the Lithuanians. While all National Revival movements looked for a medieval kingdom to claim, only Poland and Hungary sought to establish nation-state within the borders of medieval polity.